If you don't have a sewing machine, and you're interested in repairing/tailoring your own clothes (and most importantly you have the time) it's well worth looking into historic sewing methods. Hand sewing can be just as strong if not stronger than machine sewing, it just takes the right techniques and the time and patience to sit there and do it.
I've found the two most useful stitches I've picked up are the felling stitch, which is great for hemming and finishing seams that unravel; and darning for filling in holes, because instead of adding a patch you are essentially re-weaving a small area of fabric.
Proper denim jeans are quite hard to mend though even if you know what you're doing, especially if you've worn through/blown out the inner thigh region. Wherever you've sewn will become a new point of stress on the denim and it is likely to just pull away the weave of the fabric from that point.