Use some discardable material to make a test model in order to see if this helps to fit better. There are two or three ways to extend the length of the crotch. One you might try is to increase the length an inch or two by extending the waistband area beyond the pattern making sure to follow the direction of the seam, then make the test model and try it on to see if this fixes the problem without distorting the design. You might want to measure from the waist to the crotch by measureing while sitting down and adding an inch to your measurement. Another way is to cut the pattern, most have some sort of midway line along which you can cut, then tape paper to breach the gap from pattern peice bottom to pattern peice top and tape. That way the shape will remain the same, and make a model for fitting. The last way is more difficult in that one can recut the pattern crotch. Lots of error can happen doing this and you don't want to destroy your pattern. I'd advise trying the simplest first, then the second if that does not resolve the problem. There are some patterns with directions on how to make alterations on patterns, check your directions to see if there are any tips/guides etc. Hope this helps and resolves the problem adequately.
