Simplistically all you need to do is replace lost calories. You maybe burn 1500-2000 calories a day normally. You "might" burn an extra 200-250 an hour walking. So walking for seven hours solid "might" roughly double your calorie need over the day not which all needs to be replaced during the actual walking. Or it might be less than that.
Replacing that number of calories with "artificial" sources like tablets, gels etc is unlikely to be very satisfying over the three days. Better to focus on proper food - good breakfast, main meals and snacks as said like cereal bars, sandwiches, fruit etc.
But there are two angles to this - one is having the fuel (calories) to complete , the other is having the fitness. The fuel wont compensate for the fitness.
As others have said GFA is an entirely made up construct that has little real relevance beyond gaining entry to one particular race or the other. To get a GFA time you don't have to reach a certain pre-set standard or level and races vary quite considerably on their GFA times depending on who they are trying to attract.
A word of caution. You appear to be falling into a common trap that its all about running faster. That's actually probably not the best way to either lose weight (although weight loss is far more about diet) or get better as a runner. The fact that you had to stop on your 5km run suggests you are running it too fast. The risk is you will not improve in optimal way and heighten the risk of injury.
I think the issue for you is that you were advised to buy them for day to day wear as you have a very specific problem. Your needs for running may be similar or may be very different. Personally if I had a known issue that the shoes impacted on I would get them fitted again - but most decent sports shops can do this - why do you think it will be expensive and only in London?