Nowt to do with anything really - but I once did over 600 miles in a day on an RD500. That was between Milton Keynes and Cornwall - seem to remember going through Marlbrough a number of times and Frome appeared more than once at some hours distance. Ohhh I was having fun. A cracking blat for about 30 mins with a Lotus Esprit reg BUL 11T...
Mind I had a young and pliable bum then.
Best of luck on the rally, hope it all goes well for you and stay safe, would prefer to have to take the P because you did not finish due to tired etc, than did not finish for chucking the bike away.
Still you will finish and then bore us all with the tale, many times.
R