Try starting with undyed leather. When dying don't colour evenly, leave some areas lighter. Cotton wool is good (better than a brush) becuase the areas can be softened. Then use another colour to give contarast to the lighter areas. You can try brown and tan to good effect. Also rub some colour off with steel wool. This gives more contrast and also scuffs the leather for a worn look. If you take off too much colour retouch with more dye. A touch of green with the browns can also look good
For making the edges look old you can scuff them with course sand paper, a file and steel wool, or work them to give creases. Finish off with several applications of a good beeswax based leather food. If too shiny rub/ steel wool more in selected areas. If you want the chaps to look like you have ridden through thorns scratch them with a sharpened nail, screwdriver etc, Pay attention to areas that would naturally be prone to rubbing, and make these more worn, faded or lighter.
Running a marathon in them would certainly make them look old and sweat stained in no time!
Please post a picture of your finished work.