In response to the (unasked, but should have been asked) question "why should I join the TRF?" there are a few pretty simple reasons why you should.
1. Runs will be on legal routes and the run leaders have usually been doing them for years, know the best ones and can take you round a good route with minimal wasteful road work.
2. It's a national network that will help you ride in other parts of the country if you want to. I regularly ride with Devon and Herts, and could do several other areas if I got my finger out and asked.
3. Normal TRF max group size is 6 riders so as not to stand out too much to the public. You do get smaller groups and sometimes only one of two going. If you get to know them you'll know who knows the way round and can speak to them privately and go for a run that's not advertised as a group run. Hell, in time you'll get to know the lanes well enough that you could be a run leader yourself. It's about networking and working with people, simples.
4. TRF groups normally run easy/starter rides as well as more challenging. "Challenging" normally means the lanes themselves and fewer breaks rather than pace. Again, you get to know who prefers the gnarlier stuff but it's not hard to find more moderate rides in these days of forums and social media.
5. As mentioned above, the TRF does a lot of work fighting closures at both local and national level, and a large part of your annual membership fee goes into the fighting fund, keeping the lanes open for you.
Find a decent field somewhere, set up a track and attack it.
I find myself riding on totally new trails when i go off trail alone which can be dangerous sometimes, why i'd advocate for not doing it alone. When you are out there alone it's also so easy to lose track of time, but that could as well happen when you are with other people.
IME no, not good for just a couple of hours. When I was Greenlaning with 'LordLucan' it was an all day affair, though typically that included oil/filter change at the start of the day and a jetwash clean up at the end.
Lack of time was my main reason for stopping, favoring a road bike instead where I could just go for an hour blast.
Spending time with TRF was good along with the throttle monkey/muddy monkey boys.
I'd like to do it again sometime, maybe when my boys are older. Bluetooth comms will make a massive difference, so much better than walkie talkies...