Welcome to the dark side Dave...it's much more fun
There are 2 Forests in the Lakes with purpose built stuff for MTBing.
Whinlatter has recently been finished and is pretty good. However, it's also quite challenging and I wouldn't recommend it for novices. There is a good visitor centre there with Coffee Shop and lots of guides and you could just stick to the fire trails (Forest Roads) but they're quite boring.
http://www.forestry.gov.uk/alturatrail
Grizedale Forest is also laid out with a variety of trails and is much more suitable for first timers or novices. There's a good Visitor Centre, bike shop, bike hire and Coffee shop. Lots of walking trails (try the wood sculpture trail) and the purpose built MTB trails can be mixed with the Forest Trails as you want. There's also plenty of natural stuff that drops down to Coniston if you're feeling more adventurous.
http://www.forestry.gov.uk/website/fore ... nfd-5k2c5b
There's obviously stacks of other stuff in the Lakes on natural trails but much of it is very tough. There's plenty in the above locations to keep most levels busy for a day or two.
Have fun!