Biking 5 miles is sort of like walking 1 mile in terms of necessary exertion. People who take public transport are often doing this kind of thing anyway. Few of them are changing all their clothes or worrying much about odor if they just shower before they go. Take it easy on the bike, and five miles is normative activity for a short time. If it is truly flat, then cycling will actually cause less sweat per time than walking in hot weather because of the breeze created. Cyclists experience this when forced to stop is when it actually feels hotter. In a hot day, I may sweat more walking to and from the grocery store close to work at lunch, than riding in in the morning.