This thing is so individual that nobody can tell you how you can handle the biking. I would say that the best may be to borrow a bike and try a ride of that length and see how your muscles feel the next day. If they are not too sore, your body may be OK biking several days in a row.
Of course, the danger of borrowing someone else's bike is that it may not fit you properly and it may not be the setup you'd like, so it could
contribute to discouragement. My opinion is that biking is so much fun and is so contageous, that dropping up to a thousand bucks on a bike (which is not that much, btw) is fine even if you're not sure if you'd like it for commuting. I agree that going at a pace that you find relaxed
is very important. Trying to keep up with other bikers or trying to appear "stronger" could ruin the whole thing.