In reply to MisterPiggy:
So, I was out in the forest today for the first time since I posted my query. Thank you all for your wisdom.
I hadn't thought that sprinting would use different muscles from just running - naive of me. I'd not sprinted for years, so yes, shouldn't have been surprised that I couldn't, but I was.
Anyhow, I thought hard about all your comments and did the following.
Legs were feeling good so initial jog/walk/huff/puff was reduced to 1km before settling into my usual fast plod through the mud and leaves. Baloo pleased to get going earlier than usual.
About km 4 feeling good and warmed up, so keeping same cadence (as far as I could tell), lengthened my stride and really paid attention to 'form'. This was on a flattish, dry-ish part of the route. I run with a heel strike, so was really trying to 'pull' myself forward when the shoe was flat on the path. I can't really 'push' off at the end of my stride cos I don't have any muscle in my right calf; pushing off hard on the left would make me twist rightwards on every other step, which I'm sure isn't good for my skeleton/muscles. I held this new stride for about 400m: reaching forward (I could feel the stretch), pulling through (and could feel the tightening in my thighs) and trying 'run tall' and breath steadily. Felt good.
On all the uphills - of which there are many - I increased cadence; on the three really steep inclines, I increased the power - using thighs. At the top of those three sections I was really gasping.
I won't know until next time on the same route if I'm any faster, but it did feel 'faster'. I reckon the long-stride section saw me doubling my speed.
In a few days, I'll try some interval runs on a tow-path. Maybe with a stopwatch to try and be a scientific about all this.
I'll update in six weeks or so, and share any insights gleaned.
Thanks again for all the inputs.
PS. I started bouldering in a gym about two years ago, and was dumbstruck when completely stymied by the many dyno problems that I could just not do. I even made myself a t-shirt with a cartoon brontosaurus at the bottom of a cliff, with the caption: "Daddy don't dino". Now I realise that sprinting/dyno leaps use different muscles than those used to cycle/plod around a forest - doh ! "Use it or lose" has never sounded more apt !