I'm training for a marathon mostly on a treadmill. The reasons are that I tend to do much better injury-wise on the treadmill than road/trails. I have bad ankles so the uneven footing of trails is tough and the road gives me overuse injuries. The other reason is that I live in the south and it is freaking hot and humid outside.
But I realize I can't do all of my runs on the mill if I want to run an outdoor marathon. I need some road training. The question is which runs should be done outside? Long runs? Tempo runs? MP runs? Easy/recovery runs?
I can probably do one or two runs a week outside (six days of running a week, 60-70 mpw).