So I may have stupidly signed up for a 100 mile tt
That's not stupid. Forgetting your cycling shoes and doing it anyway stupid.
Which is a higher priority the mara or the 100mi? That's how you should work out how much time to spend on training for either. I would be on the TT bike doing 100 mile rides and speed work but if your priority is the run than just ride your TT bike as much as possible and try and eliminate any position issues before you hit the start line (speaking from experience here).
That's not stupid. Forgetting your cycling shoes and doing it anyway stupid.
Which is a higher priority the mara or the 100mi? That's how you should work out how much time to spend on training for either. I would be on the TT bike doing 100 mile rides and speed work but if your priority is the run than just ride your TT bike as much as possible and try and eliminate any position issues before you hit the start line (speaking from experience here).