After all, he had a very fulfilling post - chemistry teacher and principal teacher of guidance - at Paisley Grammar School, where he had taught for the previous 23 years, and where his involvement in a wealth of extra-curricular activities over the decades - Scripture Union, football teams, drama productions, pupil dances, the list goes on - seemed to come second only in importance to a never-ending willingness to lend a pastoral ear to any pupil (or sometimes member of staff) in need of quiet, measured advice on this problem or that concern.
He was, in every respect of the word, a kenspeckle Paisley figure, for whom caring relationships with students under his charge were a long-term consideration, a fact borne elegant witness by the large percentage of former pupils who still make regular contact to renew acquaintanceship and catch up on news.
If they want to do so these days, of course, it's a bit more of a long-distance affair than dropping into his Paisley home; unless they happen to catch him on a summer holiday visit, they'd be advised to try to find him in Tabeetha School, Israel, where he's currently in the second year of a four year teaching appointment.
As changes of life go, it's been pretty dramatic, and owes more to a letter from Strathclyde Region than any considerations of male-menopause:
"It was in April 1993, three months after my 50th birthday, that I found an open letter in my pigeon hole, inviting me to apply for early retirement, applications required within two weeks. My initial reaction was disbelief, followed by disappointment that the education department considered me surplus to requirements, and I was upset at the impersonal way the offer was made. However, as a Christian, I knew that my life was in the hands of a higher authority than Strathclyde Regional Education Department, and after prayerful consideration I turned towards acceptance."
The impersonality continued in the next few hectic weeks, and it still rankles slightly that the entire process was conducted by means of forms and telephone: "Not once did I actually get to speak with anyone face-to-face about the prospect of retiring early from a profession to which I had expected to devote myself for the rest of my working life."
By the time the decision was made, however, there was little time for further discussion: "On the day - a Thursday in June - that I took the short walk from Paisley Grammar School to the Education Office to hand in my acceptance of retiral, I had no idea what turn my life would take.
"On the same day, I telephoned the Church of Scotland in Edinburgh in response to a TES Scotland advert, and enquired if I had the sort of skills they might be able to use. They were having interviews for some voluntary posts on the next Tuesday and - after a frantic welter of faxed application forms and references - I presented myself for interview and was offered a post on Wednesday.
"So within one week of retiral, I was invited to travel to Israel to work as a volunteer at the St Andrew's Memorial Church and Hospice, in Jerusalem, whither I departed at the end of August."
The impersonality of retirement was softened somewhat, McLean concedes, by the considerably more personal letters of gratitude and best wishes which he received from his divisional education officer and also from Frank Pignatelli, Strathclyde's director of education. But by the time these arrived, he was in the middle of a clear-out from his laboratory and his guidance office.
"The goodwill demonstrated by pupils and staff in helping me during those weeks was enormous," he says, "and it was clearly an emotional time, but from the very first I felt very much at home once I got to Israel, where I was greatly helped by other ex-patriate Scots working at the hospice, by the staff of the hospice itself - mainly Palestinian Arabs - and by the Christian community in Jerusalem."
The decision to work voluntarily in the hospice (which, in Israel, is a kind of guest house for pilgrims and travellers to stop off and rest, rather than a hospital for the terminally ill) was based on McLean's belief that he should give part of his life and his talents in service. "I'd had just over 20 years of paid employment, so the intention was to offer two years and then see what happened, with the possibility of further voluntary work either after that, or maybe later on in the future. "
McLean travelled around Israel during that first year, including a trip to Ramallah on the West Bank, where he visited a Christian Arab school. "Palestinian Christians, besides trying to develop a working relationship with Israel, are also faced with increasing pressure from militant Islamic groups. I was impressed by the attitude of the director of the school and accepted her offer to teach science on a voluntary basis when my year at the hospice was completed."
If he thought by now that he had his future sorted out, McLean should have known better. In November 1993 he met Sharon, a nurse from England working in Jerusalem, at a Church house group. They were married on August 1 the following year,
after which they travelled to Ramallah where McLean fulfilled his teaching year, at the same time applying for selection by the Church of Scotland Board of World Mission.
In the summer of 1995 both were commissioned to work in Tabeetha School, Jaffa (McLean in paid employment as a science and chemistry teacher, Sharon on a voluntary basis as school nurse and kindergarten assistant), where they have been extremely happy, and where McLean has clearly been able to influence the lives of young people for the better in the same way as he did for 23 years in Paisley.
"The school - which is the only one in the world run by the Church of Scotland - is special in many ways. I've got two girls in my fifth year class, one Muslim, one Jewish, who are inseparabl e buddies in a way which would be inconceivable in other parts of Israel. We have Arabs and Jews, Muslims and Christians learning together in an incredible atmosphere of peace and goodwill."
By any accounts, it's been an eventful three years for Bob McLean. The stories, the experiences, the people he's met have brought a whole new meaning to the phrase "change of life".
He has a quietly reflective account of the time when a young man and woman arrived at the hospice looking for a room on Christmas Eve, "but I had to turn them away. There just weren't any available rooms. It certainly brought a touch of poignancy to the watchnight service in Bethlehem that night."
And he clearly has stories of political and human tensions in Israel's daily life which must make previous staffroom complaints about pupils, or about conditions of service, seem fairly minor in comparison. And he could tell you about - well, many more events, many more horizons which have been opened beyond his wildest imaginings of 38 months ago, and his first thoughts of early retirement when that letter appeared in his pigeon hole.
So would he recommend it to others? He shrugs. "I can't answer that. For me, it has clearly been the most wonderful time, but your motive for retirement has to be right. Like the National Lottery, many people dream about it with no real thoughts of the implications, or of what they'd really do if it happened to them.
"And for many people, if they've nothing in life to take the place of their work, then the long-term consequences could be disappointing."
But for him, it seems, the decision has been right. I wonder if he harbours any regrets. Bob McLean smiles, and I realise the question was supremely, absolutely, and comprehensively superfluous. I should have known before I asked.