The OP was asking about training runs. If he is training for ultras then he'll want to train himself to burn fat, in which case he shouldn't eat to start but only when he he feels his power is waning.
I'd disagree with that.. if you're training for ultras, long training runs are the time to get your feeding strategy nailed and to experiment with what foods work for you and what don't. Attempting what is effectively "bonk training" is ineffective, outdated and, in a fell environment, potentially dangerous. As long as you're working at a low enough intensity you'll be developing your fat metabolism but it's not an on/off switch so you'll still need carbs. The other big problem with running yourself so low is that recovery from the session will take far longer and your training the following week will suffer. Finishing a good ultra training session you should feel strong and as if you could carry on, consistent nutrition is key to this.
Spot on Nik and Daz. Couldn't have put it better, having found it out for myself.
I've not quite got the food intake right for this length of run yet - I still seem to bonk every so often so I'll try the half hour alarm suggestion. I find that some real food in with the gels does help though.
I have got my fluid intake sussed though and usually manage neither to dehydrate or to need excessive pee stops. I followed Anton Krupicka's advice which is three sips on the drink every ten minutes. I usually take three sips every time my GPS tells me I've done a mile (more often if I'm climbing, but then it's easier to remember to drink when you're hiking up hill, and easier to do it too).
Jim
I found doing long (8 hour plus) training runs through the winter last year that real food was the way to go.... pitta with houmous, miniature pork pies, sausage rolls and cold pizza all did the job. Obviously not good if you're pushing hard but for slow and steady they work a treat.
for me this would be way too little, especially in the winter when you're burning up calories at a much higher rate
if you got up at 8am, had breakfast, then sat at home for five hours, you'd be having lunch by then wouldn't you
so to run for 4-5 hours on a bowl of cereal isn't going to work for the vast majority of people, in my opinion, etc etc
As every ones says little and often and earlier than you think, Just think what calories your burning. If it's a race even more.
I'd have a slight disagreement with flopsy (Sorry) a couple of JB or half a biscuit on a 4+ hour race and I'd be joining the also rans, you need to match your energy usage. I'd always take more and not simple sugars. JB are ok for a quick fix butb unless you top up with somthing more complicated you will ground to a halt (i would)