Senior Olympics

Hi All,
I entered the local senior Olympics and did my first training ride this
morning. I ran out of steam after 3 miles as I was pushing it pretty
hard.I'm entered in the 10K race on Aug. 7, so need to energize quick.
Problem is: I've been on a low carb diet for the last 14 months. I lost
70 lbs during the first 10 months and am just trying to hold the weight
I have now.
I have been eating about 85 carb grams a day but today proved I was
doing something wrong.
Do some of you that do a little racing have any advice on what can
energize a 70 year old guy for about 30 minutes? I don't want to start
gaining weight again so a real high carb diet is out of the question. I
did try an energy candy bar but got no help.
I am in fair shape as I ride my Tour Easy about 150 miles a week and do
another 30 or so on a mtn bike.
It's the fast push that drains me so I need to build up a reserve of
energy by race day.
I do have a wager on myself with an old friend that's just 67 years old
riding a road bike.
Hope to get some help.

Jerry