Not so bad as all that DJ, I regularly rode from the train station and took around the hour mark to get to Lydeard Hill car park, though i may press on a little keener then others, even with the trailer and panniers loaded was never more than 1hr 20. A journey of around 9 miles via Kingston st Mary.
Like most places a guide is preferable for day trips when you just want to be riding and not guessing where to go, though with good use of a OS and some key pointers of where to go, you should be able to work out a loop.
Be prepared for a tough ride, as the ride up there will certainly warm you up, the tox themselves are fairly uppy downy and you'll need a little left in the tank for the ride back, it's not all quite down hill..
Depending on what sort of riding you want, the trails to the north of he crowcombe to stowey road are heathland and woodland fun swoopy trails. Where as most stuff south of there specifically in the area of cockercombe and slades are more DH orientated fun, more technical than the northern hill, expect slick, steep off camber rooty chaos.
This means if you're not keen on DH style riding, the trail type riding is a little further on meaning off road miles will be around 18-20 miles with about 3500ft of climbing in that distance. That's the bulk of your mountain biking during the day.
The 9 mile road commute there, can be made a bit more fun on the return leg with an off road BW descent, this is around a mile long piece of singletrack which takes you down to a place called Cushuish.
http://www.endomondo.com/workouts/v2Yb1I0JsCo
This is a scenic route taking a few of the trails Stert, weacombe, slaughterhouse, hill fort, holly bush and willoughby's.
It doesn't take the off road route back to town, instead retraces the route up there.
That's a 4 hr ride time and at a steady to fast pace.
Looking through all my logged routes since April i realise now how few i've done suitable to share..
Have a butchers at that route and if you need a guide i'll see where i am and what i'm doing, i'm happy to ride up from the station with you.
Oooops, 45 mins of trawling through my rides and a cuppa or two and it seems you've had some replies.