I'll post what I can,
May sees the Colne Valley MTB Challenge. A good tough one run by Parkwood Scouts Golcar, Huddersfield, getting on for 30 miles and a lot of climbs. Link here
http://cvmbc.co.uk
That site links to several other challenges, Pennine X
http://www.penninex.co.uk/, Calderdale MTB marathon
http://www.st-pauls-scouts.org.uk/
and the Holme Valley Challenge which is in September
http://www.bikechallenge.co.uk/ 35 miles this year, as good a test as you can ask for.
Pedalsport club runs a charity ride to raise funds for Overgate Hospice, Elland, a 20 odd miler with some great climbs, a brilliant moorland section , and this is run at least twice a year, next one on 19th June 2011.
http://www.overgatehospice.org.uk/fundraising/events-calendar/
Otherwise look at the Pennine Bridleway site which lists a great selection of mid distance day out rides and of course rthe infamous Mary Townley Loop
http://www.nationaltrail.co.uk/PennineBridleway/article.asp?PageId=3&ArticleId=37
Hilly Hebden is a great muddy day out in ColinJ's back garden!