KK,
Depends where you are on two spectrums, body type and age.
Body Type - some are naturally better at short and fast and some are better at long and slow. Doing the opposite to your natural ability is harder.
Age - The older you get the harder it is too improve at short and fast and the bigger the risk of injury is. Simply we are less elastic as we get older, I am sure there is a granny that can put her toes in her mouth still but I have a 15-month old that can do that for ages and giggle at the same time. In general we are less elastic and therefore training for short and fast is riskier. Plus improvements are small and it seems easier to improve at long and events.
So for many of us training and improving at short and fast events will be harder than long and slow events.
Personally I have done lots of ultras, 3-road marathons, one IM, a few half IMs (my preferred I think) and the only real injury I had was achilles following a 10km training plan in the off-season. Too much speed work.
Having said that I am itching for another go at a sprint Tri at the moment.
M.eface
