A little OT, but you also might want to consider using a heart rate monitor. This lets you know how hard you are working. Sometimes what you think is a "light ride" has you working much harder than you think, which in turn may lead to bonking faster.
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Life is too short not to ride the best bike you have, as much as you can
(looking for Torpado Super light frame/fork or for Raleigh International frame fork 58cm)