Yellow nails and nail thickening may be due to various causes, but fungal invasion is found in more than 75% of all of these cases. If your nails are both discolored and thick, it should be assumed that you have nail fungus until proven otherwise. Other causes of nail thickness include mechanical trauma about 25% of the time (the first and the fifth toenails are very frequently distorted by chronic shoe pressure) and random systemic causes 5% of the time(like psoriasis).
You may try to find solution to toenail fungus problem at this link —> Toenail Fungus Treatment
For all runners, fungus free toenails are not only more aesthetic but also require less maintenance additionally to regular bath.