Splash Mountain is in Frontierland, which is quite a hike from Tomorrowland. If you mean Space Mountain, it'll be pretty simple. Just take a right from the hub, grab the pass, then go on around the path by the race car track and you'll be in Tomorrowland in a few minutes. They're right next to each other.
If you mean Splash Mountain, you'll have to cover all of Frontierland, Adventureland and Liberty Square to get back to Tomorrowland, but it can still be done.
One thing you may want to check before making the hike - ask a CM on main street if they're even distributing FPs yet. Sometimes at open, they wait an hour or so for the lines to build before even turning the machines on. Also, certain times of year, Frontierland and Adventureland open later than the rest of the park. Just ask a CM (at the times board on Main Street would work) before you make the trip.