I’m training for a December marathon and have been getting in my long run on the weekend, the schedule I’m using has two 90 minute runs during the week, is there much of a physiological difference in breaking this into an a.m. and p.m. workout? Thanks
If you’re training seriously for a marathon I think running more than 7 times a week is inevitable but that’s not to say that you should shy out of a planned 90 minute run. There’s are many difference between a 90 minute run and say 2 45 minute runs and if you trust your programme then the 90 minute length wasn’t chosen at random so you should try and go for it. I’d suggest that you’re not just training your legs and ‘getting the miles in’ but training your body to fuel itself for a certain period of time and your mind to help you deal with the discomfort of going for such a long time. If you don’t ‘feel like’ the runs now maybe that’s a sign that sticking to the programme is just what you need 
Best of luck.
If you are going to split it due to time constraints, well, that is fine. Better to get the time/running in. Otherwise, what the other post says is correct…there is a reason the program has a straight 90 vs two 45s.
OT…split runs are usually a normal part of a serious marathoner’s protocol. Its fairly hard to run more than 50 miles per week without running twice on some days. When I go after a marathon seriously I pump my mileage up to 70-80 and I’ll have at least 3 days of double runs. Of course the long run ALWAYS remains…
According to Pfitzinger, you shouldn’t be doing doubles if you are training for a marathon until you are at or above 70 mpw.