My dd always had alot of activities before she started preschool (gymboree, art class, and 2 weekly moms groups get togethers, and a regular playdate). So when she started 2 day a week preschool (also 2 1/2 hours long) we also let her do gymnastics and swimming and kept up our playdates and some moms group activities. It just didn't work out for her. She started goofing off in gymnastics (which she had loved before), and not paying attention in swimming (which was her very favorite activity) or actively participating in preschool. I cut out gymnastics, pulled back on our moms group outings, and made our playdates more casual (playing at home or at the park vs. zoo outings, etc). Major difference. She started thriving at preschool, and loving swimming again. This year she is 4 and will start 3 day preschool, and she will also be doing ballet. I'm a bit nervous about even this schedule to see how she'll react, but she loves ballet so I'm gonna let her continue. We have 4 other 4 year old girls and one 4 year old boy living on our street so she gets plenty of socialization. She just seems to do much better with lots of downtime, and time to just play, ride bikes, run around and be silly. Even when the classes are only 45 mins. its amazing how more free time you have when you cut some of those things out. Drive time, prep time (changing between swim suits and leotards, etc), etc take a big chunk out of the day. Not to mention the kids tend to be tired afterwards. I have the opposite problem with my younger son though. He is very ready for more activites and I just can't find any that fit in our schedule (cause he's young enough that I have to participate too, which is hard when dd isn't allowed to).
I do have plenty of friends that have their kids days all scheduled up with classes and activities and the kids love it. It's just whatever works for your kid. I say give it a try, but don't be disappointed if you have try out a few things (changing times or activities, subtracting activities, etc) to find the right balance for your daughter.