@Coconut27 - thank you for letting me know your trail works, it helped me persevere with my issues. :)
Turns, out - after LOTS of testing - it wasn't mods after all. Even without mods folder at all I was having the same issue. So stuck all my mods back in (I had been careful to follow Luth's thread and only installed the ones that had been updated.) This time, tried from a different lot, and voila! Worked! Power walking they went.
Turns out, it was because I had bulldozed the Recreation Centre and changed it to residential. I do that in all my worlds when I first get a pack, because I like to build things for myself. Residential is handy, as I can leave the lot blank till I'm ready to build something. But that seemed to have broken the trails from that lot. So I built a quick recreation centre, sent the same sims -game full of mods - and no problem. One of my sims was in a fine mood - and didn't do the power walk animation, just the confident one, and the other was in a sad mood, but she did the power walk animation. But at least they were together moving around the lake.
So next question, is changing the lot and losing function considered a glitch? Maybe further testing is required, but I'm off to play.