Forza Motorsport 4

this achievement should come naturally if you are aiming for 'legend'

things start picking up a bit from season 5 onwards, with faster races and more aggresive opponents.

as 'WifelikeDrip' pointed out, there is no way of adjusting the difficulty of the AI in world tour. They can get to be a real pain as they also upgrade thier cars and know how to use them. This can sometimes (but not often) work to your advantage as they will battle each other, especially on corners which can result in one or more leaving the track.
It can prove to be benificial to take the lower class races where available as the cars seem to be a little more evenly matched, and the opponents seem a bit less agressive. anywhere between F - D. If no events in these classes are offered then try changing car type within that group. for example if there are no races for a D class fiesta, try a D class 240sx as there may be alternative for that.

try as much as posible to use cars you are familiar with as this will help you no end. If you have trouble with some races try adjusting the dificulty on the pre-race screen. If that doesnt help then make a note of which cars are giving you grief and try using one of them. i found the alfa romeo brera very useful, especially for b class races.

throughout career mode i also chose to knock out affinity level 50 and ferrari collecter, so every time i was offered a ferrari as a free car i took it, and used ferraris wherever possible as the f50 is excellent for S class races.

finally, if a race is really getting too much dont forget, you dont need to win the race in order to progress, just finish it. You can also hire a driver to help you out should you wish. this will be the 5th option on the 'race start' screen

quick addition, for me personally, i prefer cars that are better through the corners, as the ai tend to be better on the straights.
these are the cars i found usefull throught career mode, may not suit everyone but they worked for me :)
F - Citroen C1
E - Citroen C1 (uprgaded brakes then auto-upgrade) or ford fiesta
D - Alfa romeo Brera
C - Alfa romeo Brera / 1994 nissan 240sx (both auto upgraded)
B - Alfa romeo Brera - found this to be a very good all rounder, although if you can cope with the understeer, the lotus evora can dominate B class.
A - Ferrari F430 or tvr sagaris (i choose to level ferrari up as my affinity level 50 so used them wherever possible)
S - Ferrari f50. found this to be pretty much perfect for S class, only changing to the 599 GTO now and then more just so i wasnt constantly in the same car.
R3 - either the joss JT1 or the #83 F430GT - the JT1 is better on tracks with fewer corners and more straights.
R2 - maserati JMB Racing MC12 or the Koenigsegg CCGT. always prefer the mc12 though.
R1 - #7 Team bently Speed 8. its just awesome.