Orpington to Hastings is 49 miles if you cross the M25 on the B221 - less if you take the M25 spur down to the northern end of the A21, which (many years ago) was do-able. It's not flat on the stretch around Tonbridge, which probably puts it beyond the 2 hour mark for anything other than a really fast club run, but plenty of two up time trials have beaten 2 hours for 50 miles. Maresfield and Uckfield are bypassed these days (actually they've been bypassed for the a while, but I used to go down the A21 in the 70s) so I can't see that the traffic would hold you up.