There is 20 Levels, each with 3 list of objectives per level; 1 for each playstyle - Race, Pursuit, and Drive. This give a total of 60 lists to choose from. Completing a list will earn you 1 more Rank.
Starting at Rank 1 means you will have to complete 59/60 lists to reach Rank 60. There are no exact same 2 lists and as you go further down the list the objectives get more challenging, obviously. None of the Objectives is too terribly hard.
After mission 20 use the Career Overview to switch objective lists. If it is highlighted orange then you have already completed that list, you can replay lists but it will not give you another rank for completing it a second time.
Some of the more challenging objectives are
Slipstream for X seconds - to make this easier go to a hard race and just try to following one of the lead cars without trying to boost around or destroy the racer. You should be able to rack up enough time in one race without too much difficulty.
Get X Perfect Nitrous - Interstate 4 is a great place to get this, drive along the oncoming lane to keep getting your Nitrous recharged and make sure it is completely filled before holding down the boost and do not hit anything before it is emptied.
Jump Distance, Time - The center of the map has a bridge that expands a lake with tall jump locations, get some speed and hit the ramps, turn around and hit the again, rinse and repeat until you get it.
Drift distance, Drift in 1 go - The same bridge area follow the road North for two large turns, get enough speed built up and just tap the brake LT (not e-brake X) should have plenty of distance to get this one knocked out.
Get 250k SP in 1 session - Replay the Interceptor mission "Troublemaker" over and over - just North of the bridge with the jumps at the center of the map. It has a repair station close by and getting used to this mission is helpful to earn a ton of SP.