ive managed to buy myself a decent mountain bike
<snip>
the problem is ive agreed to do the london to brighon in june
Here's a big tip - make sure you aren't riding on knobblies, get some slicks. They will make a huge difference when riding on road for L2B.
L2B isn't a trivial ride by any means - but it is achievable by just about anybody with a little application. It's more about how much it will hurt you to do it, rather than if you'll finish!
I've run training rides for people from work aiming to do L2B. We went out once a week, starting off with a five mile ride (with pub stop in the middle) and built up to doing twenty (with a pub stop in the middle) over the course of three months. Everybody got around OK, even those who had had to walk up the slightest incline originally.
Ideally I'd aim to gradually build up so that you are riding 25 miles a couple of weeks beforehand (more if you are comfortable, but don't kill yourself). A good rule of thumb is that if you can manage half of an events distance in training then you can manage the event.
Certainly don't try to build up fitness in the last couple of weeks before the event, it'll do you more harm than good. Ideally get out on rides with others, as has been suggested up thread. If you can get together with a group aiming to do the same ride, then even better!