I was never very good at PE at school and I wasn’t particularly sporty or active. Throughout my career, as a secondary music teacher and then teaching children with learning difficulties, I had always been rather overweight. One day I decided I needed new clothes, so I went into a shop in Northampton and tried on the size 16s that had always fitted me. But they didn’t fit this time. This time it would have to be a size 18. I decided I needed to lose weight.

I started running, which was OK. My husband used to be in the Army, but when we went running together, he thought he was providing encouragement, but to me he was deliberately making it look effortless.

When I reached my goal - completing the Milton Keynes half marathon - I wanted a new challenge.

At the time I was reading Jane Tomlinson’s autobiography. I thought if someone with terminal cancer could complete the London Marathon, a triathlon and the Great North Run, cycle the length of Britain and do an Iron Man triathlon in three years, I could push myself further. So I started swimming lessons, began cycling and joined Northants Triathlon Club.

I started to lose weight rapidly when I got going. One of my favourite moments was when a fellow teacher told me a pupil had asked them: “Is Mrs Ryding OK? She has lost a lot of weight.” Their response was: “Yes, I think that is what she is trying to do.”

But triathlons also became a method of separating my personal life and my day job of literacy co-ordinator for children with learning difficulties. Working with children who have been excluded from school can envelop everything and become a 247 job, but my training gave me a way to let off steam and have a mini escape.

My coach, Steve Casson, started telling me that I should try out for the Great Britain 35 to 39 age group triathlon squad. I just laughed at first. But then I thought “It only costs #163;10 to apply so I might as well.”

My first race went well, but I just missed out on qualifying. The second race was on possibly the coldest, windiest and wettest June day ever and I bombed completely. It was disheartening, but I gave the final race my all. I crossed the line and my husband rushed towards me telling me I had qualified. I was completely gobsmacked. I wouldn’t let myself believe it until I had a written confirmation. When it finally sunk in it was a fantastic feeling.

I started competing at an international level at the same time that I moved jobs. I had to ask the head if I could miss my first week because I was going to Australia to compete for Britain in the World Triathlon Championships. But he was understanding and we set a new start date. The date he thought I would arrive came, but there had been a mix-up because at that point I was sitting on the GMTV sofa talking about my weight loss and competing for Britain. It was a bit embarrassing starting a job as something of a minor celebrity, but everyone was very nice.

It’s hard to manage your time when you have 10 to 12 hours of training a week and a full-time teaching job. But I slot my training into my commute - I run or cycle to school - and do longer sessions at the weekend.

I have learnt one thing: you should find something that you find fun and works for you. It makes everything 10 times easier.

Melanie Ryding was talking to Jack Knight. Do you have an experience to share? Email features@tes.co.uk.