Want some advice improve road speed, say sub 250 marathon?

Whoa Nellie. If you can't run sub 40 for a 10K after all that trail running (I'm assuming you must have a pretty dam good base) 2:50 is likely not in the cards. You'll just have to work to find out what you can hit. Lucky you - that's the whole fun of road racing / training!

If it were me (and there were actual races to enter - f'ing covid) I'd start entering road races of 5K and racing them all out.

All out means all out - not tempo or as a workout and not just a couple of races. Learn to race 5K hard. Then go read one or two of the many books out there that tell you how to improve at 5K and 10K. Once you're on a path of improving 5 and 10K race times, maybe think about 10 Mile, 1/2 Mar and Marathon. Try reading "Road Racing For Serious Runners" by Pete Pfitzinger.

First, if you are running a 1:28 HM in training during a progression run I think that a sub 40 10k is likely. Second, I would do a 5k cycle or two and work on your basic speed. Maybe a daniels plan with lots of 200s at R pace. Get on the track and practice running fast and then give a 10k and HM a whirl after a short cycle with some longer LT work (4x2 mile etc.)

I wouldn't even think about a marathon time until you do that. 2:50 isn't even in the realm of possibility right now.

Instead of guessing that you can't get under 40 for 10km you should actually do that trial and find out where you are. There really are no tricks here. Start entering shorter races and keep doing them so you get faster. It seems to me that short reps like 200s and 400s have fallen out of favor but I'd suggest doing things like 8-10 x 400 with 200 recovery or 15-20 x 200 are really useful if not overdone. Or you can do time equivalents, say 70 second and 35 second reps. But there are lots of ways to approach this and because it's something you haven't done much of you should get a fair amount of return from whatever you do as long as it's reasonable. I think the important bit is to find an approach you'll use consistently is the key. Even classic fartlek, run hard until you're tired, recover, repeat for 30-40 minutes or so will take you a long way.

I support the advice on 5k training. Coming from trail running which is more around strength, balance, rythm, etc, running flat is all about optimal running economy and this is trained with quite fast paces, from short fast reps, longer reps, down to threshold pace/HM pace i believe. It is a question of making the muscles work more economically.

I come from orienteering (running wild in terrain off trail) which is even longer away from the road, I believe the strength and aerobic capacity from hilly terrain is good to keep, but instead of running hard which I did in the terrain, I now need to run fast (if you know what I mean)

You gotta run 5x18 if you want to run 2:55 in the 'thon. Only way possible. Take it or leave it