20 miles would be end to end and back almost, but would involve quite a few metres of climbing.
If you are thinking of doing it at the weekend, an early start would be recommended and please be prepared to slow down on the descents for the walkers and the livestock and all the electric fence gates up at the moment in the section between the Wyche and Malvern Hills Hotel. There are muddy sections but much of it is well drained.
Conservators code of conduct is here, but it’s common sense required mostly
http://www.visitthemalverns.org/fckfiles/Cycling%20code%20of%20conduct.pdf
As for a starting point, from Tewksbury, I’d park at the British camp car park, head north, cross the Wyche, climb to the Worcs beacon and then round North hill back to the Wyche- there are routes that allow there and back without much duplication- have a coffee and rock bun at the cafe then explore a loop south of the camp. Bear in mind some of the trails that look good are not necessarily bridleways, especially south of the camp. The section north of the Wyche is mostly bridleways