What Causes Swelling And Pain In Feet After Miles Of Walking?

My husband completed a 47 mile walk (22 1/2 hours) on Thursday. He had some swelling in his left foot when he finished, but because he also had two blisters he thought he was overcompensating and my have strained it. The swelling seems to have gotten worse over the past two days. It is warm to the touch and red around his ankle. He claims there is no pain; however, he has a high threshold of pain. Since the walk occurred during the day and evening hours, he thinks it's possible that he may have been bitten by ants which he has had swelling from in the past. He does not think it's a problem, but my daughter and I are both concerned. Do you have any suggestions on what we should do?