Cramping- lots of suggestions but the main one we will tell you is electrolites. In Other words-SALT. Dirty word I know but most riders here will use an additive in the drink bottle to give the essential salts that you may be lacking. For most this works.
But if you feel a cramp coming on while riding- get out of the saddle and get the cramped leg on the low pedal position and lower the heel. Stretches the calf muscle and helps to ease it.
But riding- getting fitter and more riding will help. Doesn't matter that you have a Wally Mart bike----It works. Could probably be bettered- but at least it gets you riding something. And as your first bike- it will teach you a lot. Mainly what your second bike will be- but in the meantime use your Mongoose for riding on.
And if you can pull that trailer with a full load- you are not going to stay unfit for very long. Keep riding.
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How long was I in the army? Five foot seven.
Spike Milligan