It's about 8.5 - 9 miles to Seneca Lake from the trailhead. Keeping an eye on the weather, we were making good time to avoid an afternoon thunderstorm. After a 300 foot climb to Seneca Lake, that felt much more grueling than it should have, we took a much needed break. It's a beautiful spot and a nice option for camping, but we felt good so decided to continue on to Island Lake (another three miles or so).