Now I know this is basically the same as Sidanto's solution so feel free to thumbs-up his instead, but I feel my commenting would just be too verbose to simply add-on.
So, idling at the impound never worked for me, and R* keeps removing replay and "balancing" RP to make you play longer. Well, F* them.
You will need a rubber band, a Rockstar Social Club Account tied to your gamertag, and the ability to hit a few buttons every 4 and 1/2 minutes.
First, download this race from the Social club: http://rsg.ms/33748e9
I don't know how long that will be up, but it works for me. Now, once you have that queued for download, start up a GTA Online "Invite only" or other type of private session (NOT solo) so people don't bother you.
You are going to want to go to pause -> Online -> Playlists and create 3 playlists. Add in the job you just downloaded ("race!") from your bookmarked -> races section. You want to create a playlist with 16 of these races, and save it with a name like "full". Create two more the same way, one with 8 ("half"), another with 4 ("quick"). This will enable you to pick a playlist that you can keep on semi-AFK while you watch a movie, browse imgur, play with your TA DLC settings...whatever. As each race should be 4:20 long, you can give a relatively good estimate on what you can do in the time you are allotted (ex. you need to leave for work in an hour...a round of "half" should fit in there while you get ready). There is also a bonus to playlists which I will explain later.
UPDATE: Click this link to view my playlists (requires rockstar sign-in)http://socialclub.rockstargames.com/member/rriker/games/gtav...
you should be able to download the playlist, but watch out as a lot of people have hard times using that site. It can be done, just play around/click everything till you get it working.
As for setup, most of the setting for the race are already setup for you. GTA race type, weather clear, daytime, channel X. You will need to confirm those settings, start the job on your own, keep the adder chosen, and finally start the race. You will have to do that every ~4.5 minutes as each race is 8 laps long (more on that below). A rubber band on your controller's right trigger will keep you rolling onward.About the race:
The race is forward (instead of backward like Sidanto's video), so you don't have to do anything once you hit "start race". There are little barriers that nudge you along the outside of the track, and because it is a GTA race, there are checkpoints and more importantly, repair pickups along the way. The repair pickups will fix damage to your vehicle from scraping. If you accidentally changed the race type to "Rally" or "non-contact", your car will be a little haggered by the end (unless you have full armor and bulletproof tires on a custom adder. Waste of money). If you choose anything other than the adder, you'll go too fast and wedge into the walls/railings. Stick with the defaults. It is uncommon, but sometimes you can hit the wall too hard. Just hold Y to respawn, or hold the left trigger to burnout in the right direction. 6% chance of occurrence, based on my trials.Stats
So, the RP gained scales on level, with level 50 giving up 1,750RP, and going up 25RP/race for every level above that (100 topping a whopping 3,000RP). The mechanic R* placed was the longer it takes you to do a mission or race, the more RP you get, topping off at around 4 minutes and change for races, so there is no point in going 99 laps or 1 lap 99 races. Stick to the race provided.
As you go through this and set the next race up, don't forget to upvote the race for an extra 50RP. By level 90, each race was giving me ~$5,000 as well, so from lvl 60-100 I was able to buy enough to get a tank and buzzard. Very nice. I like.
At the end of the playlist, you get 100RP for every race you completed, so for the full playlist of 16, you get a bonus 1600RP, whittling down the remainder a little bit more.MATH!
level 50 full playlist about 72 min, about 30,400RP, ~$80k (with voting, and assuming you don't level up, but you will)
level 90 full playlist about 72 min, about 46,400RP, ~$80k (with votin...yadda yadda)
tl;dr = same method as Sidanto, but lazier. Download map, make playlist, semi-AFK, profit.
Credit - Sidanto for posting earlier, Doobie_daithi for making the forward-facing map, DRObythePouNd420 from reddit for math and R* post-QA QA testing