Oh yeah that climb is a brute. According to Mr. Google, the climb is 2.7 miles long, starts at 100 feet MSL and ends at 1400 feet. So 1300 feet of climbing in 2.7 miles, or just about an average grade of 9%.
The trick is, as you've discovered, that some sections are much much steeper. The switchback turns about half-way up are the steepest IIRC.
Getting to the top is awesome, because you then get to do a 40 mph descent down the west side until you hit Coal Creek. All the roads on that side (Forest Dr, Lakemont, Cougar) have nice bike lanes and very few stop signs and traffic lights.
My loop: Bellevue to Marymoor via 520 trail, then clockwise around Lake Sammamish Pkwy to Issaquah, Newport Way to SE 54th (AKA Zoo road), over the mountain (54th, 60th, Cougar, Lakemont, Forest) and down to Coal Creek, then Factoria, and finally Lake Washington loop back home.