Another great podcast @Mikael_Eriksson 
I think the intensity is less important than the sustainability regardless of whether you are are pro or not. If you have a programme that you can repeat session on session, week on week, while not breaking down that is the key to improvement over time.
There can be a fine line sometimes between being very fit and very injured and the higher the intensity of any training often those risks of injury are magnified and that’s the balance of trying to get in the extra intensity. The question is, is it worth it?
Each of us has a slightly different balance where that is concerned but as Joel said the benefits of consistent training at a slightly sub maximal level far outweigh the risks of going slightly too hard and finding the inconsistency that may come with either potentially injury or the need for extended recovery periods. Endurance gains are a long term project that need consistent inputs.