I expect the OP has done the ride by now, but i am still a bit miffed.
Seriously, i dont understand some of the folk in this thread, i am currently 17.5 stone, i used to be 21 stone, i am not a small chap by any means, but 40 miles on road is not really a hard ride
its only "hard" if you twaddle along and spend hours upon hours doing it (then it can hurt your hoop a bit)
The missus has booked us a night away at the end of may for my birthday, the only problem is its a 70 mile ride to get to it and a 70 mile ride the following day to get back, i am expecting that to be a bit of a pain (literally) i might even fit some slick/spare/commute tyres to the 99 FSR (well she has hybrid tyres on her bike lol) seams a waste to wear down my new Conti Mountainking/Rubber Queen combo.
The most i have done on a ride so far was a 58 miler last summer, that was mostly road/path stuff around the NE coastline and was not so bad, but the missus likes to go at a slower pace so it means more saddle time (hence a sore hoop)
The missus did this 140 miler over two days last summer and she hardly rides her bike much at all (she did say it was hard for her though)