What is the event? NDW? UTMB? Lakeland 100? Trail? Hills? Laps round a track?
Best thing is to try and train on the type of terrain that you'll be racing. If you are doing UTMB or Lakeland 100 for example, you need to do tons of vertical. If possible, get out on the course and recce as much of the route as you can.
Are you planning on racing for a place, or a time, or just to get round? If it's the latter, then your training shouldn't need to be any different from your 50 mile training.
Work out your nutrition plan? What food & drink is on the checkpoints, and what do you need to carry? Do you need food that requires preparation or cooking for example, and will facilities be available?
Think about what you will be wearing. How cold/wet might it be. Are your clothes going to be OK after 24 hours soaking wet perhaps? What will you carry, and what can you leave with crew/checkpoints/supporters?
Physically, 100 miles should be doable for any long distance runner if you get the pace right. If not, then it will be a very uncomfortable experience. Practise running on tired legs if you're not used to it.
Good luck.