I don't have anything to compare it to, but at 37, I don't think I've found it too hard. Yes, I've been tired, suffered from insomnia, SPD, heartburn and acid reflux but overall I don't think I've been that bad and I've kind of enjoyed being pregnant. I think I'll actually miss it a bit!
I've not put on a huge amount of weight (yet! Maternity leave has so far involved lots of eating), 20 pounds at 36+5, but every excess pound has gone straight onto my arse. Put a dress on last night I hadn't worn before, bump looked great, arse needed its own post code 
Thanks for all the TENS machine advice. Definitely sounds like a maternity one is the way to go. If it can also be used for back pain, great, if not, tough! The cost of renting is almost as much as buying so as there is a chance it might be useful for DH and a tiny chance I might do this again (hopefully DH will be so besotted that he'll change his mind about having another), buying looks like the sensible option.