Forza Motorsport 5

You can do this with yourself and 15 AI in a private lobby.
Got to multiplayer Y for private match once in match go to options and pick a B class car (I chose B because when I used anything lower I would complete 3 laps and game would kick me out of lobby,and say I lost connection to lobby could be my connection. Try any lower class at your own risk)
Now after you select a low class car go to the race options and follow what I posted below
Game Type: Custom
Environment: Any
Track: Indianapolis Oval
Max Players: 16
Number of AI: Max 15
Drivatar Difficulty: Unbeatable
Laps: Max or whatever you like I did 25-30 it would take me about 18min-22min each race.(50 would be under 1 hour)
Damage difficulty: Whatever you like
Collision Mode: Always off (up to you I like it off)
Car class: Any
Drive type: Any

Hit RB and go down to the bottom and change
End Of Race Timer: 5sec

Now hit A for accept, next go back into change Car and select an S class car.I used my upgraded Ferrari F12berlinetta 2012 It eats this track. I will post my tune setting at the bottom :) with this car it took about 54-56 sec per lap. Or use whatever car you would like but I never did it with anything higher than an S,so it may kick you out if you do.
Select your new car and you will now see that all the AI cars are stuck at a B class or whatever low level class you chose.
This will make leading very easy :)

I also unlocked my 1000 mile this way.

I would recommend completing the 325 medal achievement 1st to help cut down play time.

Here is my cars setup:
Ferrari F12berlinetta 2012 Class S
Description: 1000miles

I also did design or paint job for it to help keep you guys going
Design/paint job
Description: 1000miles

Please feel free to use download them and happy grinding and I hope this guide helps :)

somehow this is going real fast just by drag racing.dont know how but it works.plus you can work on your medals also.I got to 500 miles in about 20 races.

I don't know if I was just lucky with a glitch but, I have tried it on two separate gamer tags and, all 5no multiplayer distance achievements unlocked in around 30 minutes on each gamertag. The other account this unlocked on was TAPres which is also on this site.

The glitch first came to light on my TAPres account after doing 3no private multiplayer drag races in a custom paint and tune set up car I had done to boost Painting by numbers and, Pro Tuner for this gamertag. After completing just 3no 1/4 mile drag races, the Follow Me achievement unlocked. Now, this was a new account that had never raced online before and, after 3/4 mile it registered I had led for 50 miles in multiplayer races. After I carried on the 10 heat race, other achievements continued to pop being :-

Born Leader - Lead for 200 miles
Eat My Dust - Lead for 350 miles
Pace Setter - Lead for 500 miles
Lead Dog - Lead for 1000 miles

All these were unlocked in 26 heats of 1/4 mile drag races. So, the game registered I led for 1000 miles after only 6.75 actual miles and about 30 minutes of game time. Well worth the 5no achievements . After each 10 heats completes, just press to restart the heats again.

To replicate what I did, I used my Mercedes A45 with a custom set up of increasing the tyre pressures. Both the car and tune which worked on both gamertags is shared online under my gamertag Presariofu. I don't know if this design/tune made a difference or, it was just the drag race parameters. For what it takes, feel free to download the set up/design or, use your own car.

I went into multiplayer and, pressed to create a private lobby. From there, I selected my car and then pressed to change the car/options. In game options I set the following :-

Game Type – Drag
Environment – Any
Track – 2-Car ¼-Mile Drag
Max Players - 16
Max AI Players - 0
Force off TCS - No
Easiest Shifting Allowed - Automatic
Number of Heats - 10
Car Class - C
Drive Type - Any

I hope this works for you as it did for me. As anyone knows, it would take some serious devotion and, a lot of hours to unlock the lead for 1000 miles achievement in the same day as 50 which you can see in my achievements unlocked.

Good luck. Please let me know if this glitch dosnt work for you and, I will take down this guide.

I've just found a easy way to get those medals:
I don't know if this is a glitch or a feature of the race, but let's go...
1. Go to a normal (not private) Drag multiplayer race.
The most important:
2. Choose a car with a different class of everyone in the lobby. Players are usually using class R, X, S, P... I chose an A class car;
3. You'll race "alone". You can be the last one to get to the finish line, but you'll still get gold medal.

** EDIT: Gustavo82RS found a pattern to race alone in a full lobby... You must have the lowest class car and have to be in a lobby with an even odd number of players. Doing it, you'll race alone and therefore, win the gold medal. Thanks man!

** EDIT 2: Translation error... It's not an even number of players... It an ODD number of players (1,3,5,7,...). Really sorry about that. Thanks IzzyXboxOne!

4. Rinse & repeat.

Using this technique you can go for all Multiplayer achievements for leading, for getting gold medal and even for affinity level, but it will take a looong time for the affinity, so I think you should try the better solutions available here.

If there's any mistake, let me know and I'll improve it.