Plenty of bike trails are available for a Maine biking expedition

45 miles of historic carriage roads donated by the Rockefeller family weave around the mountains and valleys through the heart of Acadia National Park. The roads have crushed rock surfaces perfect for bicycling. Cyclists share the carriage roads with horses and pedestrians, enjoying sweeping vistas and close-up views of one of America’s most breathtaking landscapes.
Click here for a Trail Map (PDF)

The National East Coast Greenway is a 3,000-mile biking trail connecting Calais, Maine, to Key West, Florida. The greenway route in Maine is about 400 miles. Here are some of the greeway loops. All rides below are described as moderate to difficult, for intermediate and experienced cyclists.
Eastern Trail from Kittery to South Portland. 68 miles. The Eastern Trail connects Bug Light Park in South Portland with Strawbery Banke in Portsmouth, NH.
Casco Bay from South Portland to Brunswick. 35 miles. This pleasant ride takes cyclists from Portland’s waterfront to nearby suburban towns, including Falmouth, Yarmouth, Freeport and Brunswick, and the Maine coastline.
Details of trails .

The Kennebec River Rail Trail runs adjacent to the railroad bed along the Kennebeck River. It is six and a half miles long and it is open for walking, running, bicycling, skateboarding, rollerblading, and dog walking. The terminal points of the trail are Waterfront Park in Augusta and the city of Gardiner. The trail passes through the towns of Hallowell and Farmingdale. Trail Map.
Access points to the rail are at Greenville Street in Hallowell; at Kennebec Bike & Ski, 357 Maine Avenue in Farmingdale; Pine Hill Drive in Farmingdale; Bowman Street in Farmingdale; and Sheldon Street in Farmingdale
Parking is at the MSHA parking lot on Water Street in Augusta; the trailhead under the Memorial Bridge in Augusta; Capitol Park in Augusta; the municipal lot on Front Street in Hallowell, and the Hannaford at 100 Maine Avenue in Gardiner
Portable restrooms are under Memorial bridge and at the Capital connector in Augusta; in the municipal parking lot in Hallowell

This 1-mile trail is a flat, paved surface through the Bayside and East Bayside neighborhoods. It extends at the south end from Elm Street between Somerset Street and Marginal Way and parallels Marginal Way, linking with the East Promenade and the Back Cove Trails near the highway overpass where Route 1 crosses Back Cove. There are several benches and three water fountains. Trail is good for walkers, runners, and bicyclists.

The Down East Sunrise Trail is preserving 85 miles of rail corridor for future use by snowmobilers, ATV-ers, pedestrians, bicyclists, cross country skiers, equestrians and walkers. Now, 30 miles of multi-use trail is open for permanent year-round use from Machias, east to Ayers Junction (except for mud season). The remainder of the trail from Machias west to Washington Junction will be opened in phases.
Trail map.

Plans call for the Mountain Division Trail to ultimately cover 52 miles between Fryeburg and Portland. Two sections of the trail are open now. A 6-mile segment between Standish and South Windham provides a gently rolling ride in the Sebago Lake watershed area, northwest of Portland. The second leg, in Fryeburg, begins not far from the Maine–New Hampshire border and travels 4 miles along the railroad right-of-way. To get onto the southern segment, start from the western trailhead at Johnson Field in Standish. To get onto the shorter northern segment, start from the Maine State Visitor Information Center at 97 Main Street in Fryeburg. The paved trail ends near the small regional airport and the Brownfield town line.