In terms of time, 8 miles to 13 miles is what, an extra 40-50 mins? Given that running further doesn't involve any extra time in warming up or cooling down, putting on running gear, having a shower afterwards, etc.
I agree with Archimedes - most running programs have 1 long run per week. Hopefully your partner can cover for you for 45 mins once a weekend?
The general advice on increasing distance is to add 10% a week. If you can run 8 miles comfortably, then adding 1 more mile a week should be well within your capabilities.
As a VERY rough guide, the body holds around 2000 calories in usable energy, and burns 100-150 calories per mile. 8 miles should be easily doable within your current reserves, and a full 26 miles will almost certainly require some replenishment along the way. 13 miles can have some people close to the point of depleting their reserves, while others are completely fine - so eating something along the way is probably a wise precaution, but not absolutely necessary.
Ideally you want something fairly high in simple carbs, as these are digested very easily and energy starts hitting the bloodstream in 20-30 minutes. While M&M's do contain simple carbs, a lot of the energy in them actually comes from fat (in the chocolate) which can take 5 hours to digest, which isn't of much use to you during a run. Jelly beans and gummy bears certainly fit the bill on this one, and many runners use these. There's nothing special about energy gels over more mundane sources of glucose, and if you want to steer away from them to avoid caffeine, then I think that makes a lot of sense.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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