I did Offa's from Knighton to Prestatyn a couple of years ago, kind of on a whim. I had a week of work and wasn't sure what to do with it, so I bought the Trailblazer guide and off I went. I absolutely loved it and caught the long distance walking bug.
The first day was Knighton to Mellington over the switchbacks which was tough going and something of a baptism of fire for walking with a heavy pack (something I'd never really done before for more than 3-4 miles for overnight wild camping), but I got stronger and fitter, day on day. When I got to Prestatyn I was full of beans and would have happily walked all the way back again. Sadly i had to return to work though.
Since then I've done other National Trails but Offa's Dyke may be my favourite still. There's so much variety along the route. I'm planning to return in April to do Knighton to Chepstow.