The Lewis & Clark Expedition across America officially started from Illinois in 1804
and reached the Pacific Ocean on the 554
They had travelled more than 4000 miles by foot, horse, and canoe, upriver along the Missouri River, and through Montana. They stayed at the Pacific for the winter then turned around and travelled the whole route again back to the East Coast. The entire round trip took 2 years, 4 months and 9 days.
For a great site about it, and for fascinating diary extracts, see here: http://lewisandclarktrail.com/index.html
(I'm hooked now, and have to read the whole journal
.)Edit: corrected date & added links.