Trails -v- Road

I have picked up a niggle on my foot (a stiff joint!) from marathon training - it bloody hurts when I am running on the road so I have hit the trails instead.  My only worry is that the trails I run (Sandstone in Cheshire) aren't easy and I am worried every workout is now classed as "hard".  I am planning on keeping my long run on the road but I need to minimise impact on foot, hence rest of runs on trail and it is only way I can run pain free. 

Question is - do any of you marathon train on hilly trails for a road marathon? (I am doing Manchester end of April) If so, do you just go out on trails and "run" same pace each time? Should I change mileage for time on feet? So do the time it takes me to run an easy 12 miles (around 1:50) on the trails instead rather than 12 miles...? 

Don't know, but I'd have thought you'd be OK if you really slow down on the uphills...   For example, if you need the same effort to run at 13 minute/mile pace up the hill, as you do to run your normal pace on the road, then why not do that?

Most people round here train off road for their marathons, including me. You can run easy off road but sometimes you have to work hard up hills. The different muscle groups used in a long run makes it easier on the body.

Heow - dont let it worry you. I've started using my local park to get some runs in. becuse its softer on joints. I actually consider trail running to be real honest miles becuase they make you work harder aerobically. Yesterday I did 56mins of hard hilly trails at about the intensity of a half marathon. I find doing sessions like that make the road feel easy.

But yes some of the best marathon runners train off road all the time (ethiopians, kenyans ect)

About the time vs distance issue I'm not sure if it makes any difference really. I've been just using a cheapo casio watch for last 2months or so and have smashed pbs because I've just run hard rather than to a certain pace. Old school I know but it seems to be working.