20s: Decade of invincibility
"From a physiological viewpoint, you really are at your peak in your 20s," says David Jones, professor of sport and exercise sciences at Birmingham University. "Although women reach their natural shape and size in their mid teens, men are still growing and building muscle mass in their early 20s, and will reach their optimum before they're 30.
"As muscles bulk up, the tendons enlarge to cope with the extra load. If you're training hard, your muscles will be slightly stronger than the average person's, and you will certainly have a better blood supply to the muscles. Your VO2 max - the amount of oxygen your body can consume while exercising - will be at its full potential too.
"Bone density also reaches its peak in the early 20s for men - a few years after it has peaked in women - and it remains constant during this decade."
You may start to feel twinges in your knees towards the end of your 20s. Cartilage, the gel-like, shock-absorbing substance that lines the ends of your bones, can become frayed as your 30th birthday looms.
Also, "young, unsupervised athletes usually don't get enough sleep, hydration or adequate nutrition," says Bradley Young, a sports psychologist at the University of Ottawa, Canada. "At some point, often seven or eight years after leaving school, you realise that you can't stay up until two o'clock in the morning and belt out an eight-mile tempo run the next day. You eventually learn to self-regulate to save yourself from becoming a post-education running casualty."
Your most important training tool this decade? Self-control. Cardiovascularly, you're a rock star, but your musculoskeletal system can't always keep up with your heart and your lungs. "The demands and impact of running are too intense on your joints and muscles to complete tough workout after tough workout without getting injured," says McMillan. He recommends taking at least one easy day between hard runs and incorporating impact-free cross-training activities into your routine.
"Runners in their 20s tend to either eat poorly or eat just to get by; they don't make the connection between food and performance," says running nutritionist Lisa Dorfman, author of The Vegetarian Sports Nutrition Guide (£12.99, John Wiley & Sons).
When you're running, you want your body to tap into easily accessible carbs for fuel, not drain your protein stores. "Protein aids muscle repair, boosts your immune system, maintains healthy hair and skin, and manages hormones and water balance," says Dorfman.
Before any run that exceeds an hour, eat about 40g of carbs (240ml of sports drink and half a banana). If you're going longer than 90 minutes, restock your carbs every hour with 470ml of sports drink or with a gel and some water. Within an hour of finishing your run, jump-start your recovery with a carb and protein snack (chocolate milkshake and toast, or a smoothie).
28, from Leeds
"I ran cross-country and middle distance at school, but in my late teens I stepped up to 5K and 10K. I’m sure all the running helped change my body shape from skinny to more bulky. In 2003, I did the Great North Run in 1:06:28, which was a real breakthrough for me, and I’ve now won the Midlands Cross Country title four times.
“I run about 90 miles a week and my training is very structured, but I’ve suffered with stress fractures to my foot, which has been frustrating. I eat loads of carbs, but quickly burn them off – and although I don’t have the perfect diet, I eat more good food than bad. I want to improve my PB at 5K (13:56) and 10K (29:04) while youth’s on my side.”