I think your doing a fantastic job,and sorry to hear about your father.
I'd be interested to know what bike you own? i do a lot of Mountain biking myself,great fun. I ride down at Swinley forest.
75 MILES is A Heck of LOT!,Especially for a mountain bike. Me and my friend do about 10 miles every weekend or so and it KILLS us. Its much easier on a road bike,(higher geared and no terrain,just plain smooth road
) but with Mountain biking you have rocks,gravel,mud etc etc. ITS VERY DIFFICULT!.
Anyway,i wish you all the best of luck.
Thanks.