If you want to hook up with some local rides try Summit MTB or go to the Cycle Care LBS in High Wycombe. I think they might have a link with Dees Cycles in Amersham, but I’m not sure.
If you would rather do your own lonesome thang, there are quite a few bridleways around Amersham which are good when dry but ‘orrible right now after the rain.
You could try riding south-east on the A413 towards the Chalfonts (try to pick a “quiet” time), then right into Bottom House Farm Lane. Follow this uphill to Hodgemoor Wood where there are some surfaced (for horses but good for mtb’s) and unsurfaced trails. Then head west through Penn Bottom to Common Wood where there are more (gravel) surfaced trails which hold up well in the wet. Go north from there (thru Penn Wood if you’re feeling cheeky 😉 although there is a bridleway too) and drop down the bridleway to Little Missenden (good pubs). Then up the hill on the other side of the valley and make your way back to Amersham.
You really need an OS!