So I am going to be training for  March 1 marathon.  I really need to get my steady state speed up... however, I love trail running.

Can you mix road and trail when marathon training effectively.... meaning running big up and down on a mountain (not just on flat trails) and still benefit.

I was thinking that as long as the quality days were on road (T, M, Long) the filler runs could be done on trail for miles.

Thoughts?

Since you're asking this, would it be a correct assumption that your marathon is a relatively flat road marathon and not a hilly (3000+ ft up) trail or hybrid marathon?

I would say yes, they can be mixed, but depending upon where you are, the mountains may be snow covered much of that time, which can make them easier or harder. (Our popular single track mountain trails get packed by use and may be smoother than the rutted trails in summer - unless it gets icy or deep snows, then traction devices or snowshoes get used.)

The uphill part of the trail is resistance training, the downhill is speed. If your trail isn't too twisty, you could do an uphill tempo for extra bang for your buck. (if I'm doing an uphill tempo, I'll generally be on a mountain road since the slope is something I can run - about 8% - whereas the mountain trails - 20-30% - are power hikes for me.)

A lot obviously depends on what your trails and hills are like.

A year when I was sort of training seriously...

Leadville Orientation weekend made me recall I needed to do a bit of speed stuff.

I recalled I used to use marathons for tempo runs for (ahem, cough, cough) "fast" fifty milers.

There was a flat marathon about six weeks out in front of me.

I kept all trail stuff except one night a week I went to the track or did sixish hard push on pavement somewhere.

I also ran (pavement) to the trailhead instead of driving (about five miles).

That was plenty to refresh my mind for the steady state of road stride.

I ran about 3:12 and that was plenty good for me.

I did play with speed, lots of different effort stuff, on mild trails.

When I'm training for a road race (e.g. my annual "benchmark" HM), I run the majority of miles (60-80%) on roads, or on gravel roads in the mountains. The gravel roads are remote, so they feel a bit more like trails, but they still have many real road features, like the much smoother surface than the trails (my trails are very rugged and rocky), and long, mentally challenging, fairly flat stretches. What I find quite great about them is that their hills are often steeper than hills within a town or city, so they make for good hill training.

Maybe you have some gravel roads in your mountains, too?

I think speedwork (on a track, or a measured stretch of flat pavement) can make you faster if you feel that the trails might slow you down. Running trails during a training phase for a road race relieves tendons and muscles which are overstressed on long pavement runs, they keep your joints flexible, and are a nice mental change. I always have to have at least some trail running back in the hills, just so that I don't get tunnel vision of my road race.