Downtown's not too bad, it's mostly flat but getting out to Lake Merced may involve hills. Maybe not too many if you go Valencia to outer Mission area then cut over via a south (i.e. flatter) route. It'd probably be faster than trying to go over Market/Portola or around to GG Park. That's what maps are for

Now if the "SF Roll Call Ride" would get enough people together we could have Team BF!