Well you could wear "FITTED" ankle or leg weights. The reason I say fitted, is because you do not want to cause an injury, because something you are wearing is flopping a round on your leg/ankle.
There are weighted shoes, but I don't know how good they are for doing more than walking in them.
Normally for track and and other sport than you need running speed, well fitted shoes and a weight vest help. This is a vest you wear that can have weights added or removed.
There are also vest that have a rope/line trailing behind it, that you can drag a weight. I see guys wearing these walking/running up sand dunes. Running uphill in sand weighted or not is good for the leg muscles, balance and endurance. Will also help you increase your speed as well. Also running or walking on dry loose sand will help as well.
If you have weak knees/ankles or know that your ankles turn easily, or that yur knees go out just walking, you will need to go slow and maybe have someone SHOW you how to do certain kinds of exercise and that can help you adjust your body positioning, until you have strenghtened them. A training/coach can spot bad form, where you can't.
The main thing will be to include exercises that build up (strengthen) your leg, knee and ankle muscles. A good idea is to find someone that can help you train your legs from telemark skiing. These exercises will help a lot, whether you plan to telemark ski or not.
An example of SOME of these can be found here:
http://www.bodyresults.com/S2Skiprep.asp