I think a bit of both, Syndrome. You need to increase strength in the legs to be able to increase your average speed, and you can do that by increasing your distance to increase endurance, and working to increase your average speed to build strength.
A friend of mine has been pestering me to go for a ride with him, but he has a mountain bike, and I have a road bike. He has a fear of being knocked off the road he's unwilling to overcome.
So a few weeks ago, I thought I'd buy a cheapy mountain bike off Ebay to see what it was all about. There is a State Forest only 2-3 kms up the road from my place, with heaps of maintained MTB trails.
I ended up spending $150 on a clapped out Diamondback 29" with el-cheapo gearset. At least it has disc brakes! I replaced the crank bearings with a cartridge type, cleaned and oiled the moving bits and fitted Shimano MTB pedals.
I've had a few forays into the bush now, and, Geez, it's hard work! I'm not a gun road cyclist by any means, but I wasn't prepared to be gasping for breath after five minutes! The lack of traction at times, and riding over logs and rocks is something to get used to. And there are no flats to rest on - only climbs and descents.
Anyway, I figure if I can use the hills in the bush trails to build my leg strength and lung capacity, it'll help me with my road cycling (getting up hills and being able to maintain higher gears for longer).
I'll also see if I can use my cheap as chips mountain bike to keep up with my mate on his very expensive machine.
Here's a pic of the clunker, with the smooth, finely tuned machine...
I'll keep going with it... its starting to be fun... once I get my breath!