Southwest to Walnut isn't a bad way to go. I usually use Main, which
sounds like a death-trap, but depending on the time of day, it's
really not bad. It isn't terribly hilly (given the adjacent roads) and
motorists are used to seeing cyclists there. It's got two lanes in
both directions, which makes safe passing easy for motorists. You can
take the lane.
Main becomes Delaware north of 7th street. Watch out for the storm
drain just north of Independence Avenue, though, as it will kill
tires. I really wish they'd replace it.
To get back, you can cruise down Wyandotte to get out of River Market,
then cut over on 12th or 14th and take Baltimore south. Those two
roads are seldom used by hurried drivers. Sections of them are
one-way-only, and Baltimore, sadly, is not cut through to River
Market. Alternatively, you could just take Delaware and main down to