How best to get fit quick, would ridding lots and lots be a
good idea, 4 days on the go at present and legs are tired!

Running and/or a rowing machine. Eat/drink wisely. Get enough good quality sleep.

get a road bike if you want to get fit for just riding.

Combine road bike with running if you want a bit of all round fitness.

How quick is 'quick'?

The first thing to be aware of is that you only get fitter when you recover – eg if you just train loads and don't ever recover, you'll not really get any fitter, just more likely ill/injured.

Give us a timescale and there'll be plenty of help with some sort of training guidance. Useful info would be – when, what, how fit you are at the moment, how much training you can fit in, etc.