I’ve done the Lakeland Monster the past two years – that’s 65 miles, mainly off road and with some nasty hills so not too dissimilar to what you describe…..
First year I did it on a Spesh Crux CX – it was slightly too big (it was a 56…54 is too small….and they don’t do a 55!) so my position wasn’t quite right – on top of that I had really narrow 30c cx race tyres on – – I suffered!!! Mainly due sot neck and back issues rather than leg problems!
Anyway, last year I did it on a Spesh Stumpjumper Evo 29er hard tail – and knocked 50 mins off the previous years time and felt fresher at the end!
I’ve already acquitted this years bike (Giant XTC SLR cross bike), with a proper fit, wider tyres and some nice low gearing I’m planning on going a good half hour faster still this year…..
I reckon comfort comes first, which means riding position, tyres and gearing more than suspension and weight…..that’s my twopenneth anyway!!
🙂