I've only done one 24hr race and it went really well so I can tell you what worked for me.
I ran the first 70km (not planned, just ran for as long as it felt comfortable). I then switched to my planned run 3mins, walk 2mins schedule. That lasted until I got a bad case of trapped wind at about 11-12hrs and I had to walk for about 2 hrs. Someone gave me some bicarbonate of soda, I chundered, cleared the gas and then felt fine. From then on I ran 700m and walked 300m (every 50m was marked on to the 1km track) until the end.
I think the most important thing is to be able to change your plans.
I'm doing another 24hr run in November and will aim to do a similar thing (without the 2 hours of trapped gas as I'll be taking my own bicarb of soda).
Good luck with Belfast!