As everyone is talking about convocations and med certs, may I make the suggestion made here a few years ago by someone else - scan and email both to yourself. They are so critical to competing that you cannot afford to leave them at home.
Lowrez - there is an old Comrades saying oft proven true - for every minute you bank in the first half, you'll lose 6 minutes in the 2nd half. Pick your pace - figure out when you have to be where - and be religious to it. If you've still got lots of steam left with 15 miles or so to go, do whatever you want. Lindsey (Official Comrades Coach) says he gives permission for his runners to only go all out when they reach tollgate (on the down run) - and that's only about 5 miles from the finish!
We did do a p park run before Comrades, but not asa race - a more gentle pace. But I think before a marathon that you intend to race, it's a different story. I think it's a good idea to do the 5k, and to really race it pretty fast (not all out). Why? - after the taper you need to wake up your legs, get them used to a fast turnover. But you need to be careful - don't trip. 5k will not endanger your nutrition stores, nor tax your muscles to tiredness.
Med certificate: even though they barely glance at it, I wouldn't risk a certificate with anything crossed out.