Welcome, Glaconman, Great news about entering the Liverpool Half. Getting to the finish line takes courage and determination, but believe me, for a new runner, getting to the start line is an even greater achievement. Congratulations.
I wouldn't worry too much about impact injuries and stress on your joints. It's a self-evident truth that joints will suffer more from hard surfaces than from soft, but unless you begin running with weak or vulnerable joints, I'd say you'd have to be pounding the pavements for many years before your joints start to complain seriously.
If you are worried, bear in mind that there's quite a difference between concrete and tarmac. So, when circumstances allow it's better to run in the road than on pavement.
A bigger worry, as I've discovered this year, is that a poor warm-up AND/OR insufficient stretching AND/OR inadequate footwear AND/OR a too-rapid increase in weekly mileage, is FAR more likely to cause injury than hardness of the surface. I recently pulled a calf muscle that kept me out for a month, and that was done on a soft, cindery canal towpath. That said, I agree that in general, it's much nicer to run on woodland trails than roads.
If you're preparing to run a half marathon you should have some kind of a schedule.
http://www.halhigdon.com has a Half schedule, and you'll find others if you do a web search. I think mileage is more important than the running surface.
If you prefer running on softer surfaces, then go ahead and do so, but it might be worth ensuring that you do at least a 10 miler on the road before the big day. Does anyone else have anything more useful to add?
Good luck with the race mate, and be sure to report back on your training and/or the event itself.
Andy