Mmm tricky one Dominic. If you had or have a coach this would be definitley one for him or her since they would know your trainining history, how well you recover and the type of workouts that give best results etc. Things worth considering:
1. Current training status (are you race fit or not?)
2. How well do you recover from blocks of training? (especially from race pace efforts)
3. If time is limited 2/3 near or around threshold pace efforts (ideally on your TT bike) seem the obvious route to take since your event is only a few weeks away
4. Sometimes to get yourself really fit you may have to risk some degree of fatigue through workouts and then look forward to improvements when you're refreshed and recovered.
5. I think the general advice is (hopefully Huw, Paul and others will chip in) is to taper more for short endurance events (Pursuiting etc). However for longer events such as 50's you don't want to take lots of time off approaching your event because this may have a detremental effect. I'm trying to get some good form for the ACC Road Race (April 4th) but I know I won't be at my best cause its still early in the season and historically I don't start doing well till late summer so i'm gonna see if I can peak early and late
Good luck
