How do I start a long-running game?

Do you mean setting aging to long?

I mean, I can't seem to get over my need to be a perfectionist. I can't start a legacy because I get bored and end up ageing everyone up by like 3 gens.

I guess it depends on the characters you make, I'm on gen 12 (I think) and always got bored a few gens in. Ths time I've made inheritance rules and stuff so my next gen aren't rolling in simoleans. As well as rules like no painting unless in the painter career, no gardening unless in the gardening career. It's made it last a lot longer. Huber Family Current Tree

For me the biggest key is having well-defined characters and creating my own stories. Otherwise I just go through the motions of playing the game and eventually get bored.

Try rotational. You get bored with one household you can simply switch to another. Make them diverse (different life states, singles, families, different goals for each of them, in different neighborhoods, etc.) so it's not same thing, different family. Maybe have their lives intermingle & create some drama between them.

I discovered I was making the same sims over and over. I made my own sort of challenge. I rolled dice to get my sims traits and story. I made a list of milestones and when she reaches one I roll the die again. For example my current game was a homeless teen. She had to earn enough money to have a small house before she could get things like a bed, fridge or toilet. She had to earn an A grade, age up to young adult and have a small house before I rolled again. The next story she rolled was twin siblings. So she had to earn enough to add a second bedroom and furnish the room to gain custody of her siblings. She is trying to give her sisters the best possible life so all holidays are done with great enthusiasm. She rolled the Music aspiration next and per my rules she had to move to San Myshuno to pursue her dream.

I am having a lot of fun with this because even I do not know where the story is going to go. I hope I can keep it going for several generations. The story does not get stagnate because it is constantly changing and evolving.

How do I start a long-running game?

You open up any game of your choosing and leave your computer on for a year.

Joke aside...I really admire those who can stick with a family for generations. Perhaps doing a challenge can help you, but I can't really advice you well, since I tend to lose focus quickly as well. The households I want to keep playing, I take a break from now and then. But more importantly is probably that you accept yourself and the fact that not everybody has the stamina and patience for long-running families. I know I have had to.

I run only legacy families. The first thing is have a concept. Have a goal for your starting sims. A story behind them, why they are there at all, where they are going. You need an emotional connection with your sims really. They can’t just be dolls you move around. Finally, patience. Rome wasn’t built in a day.

I find it's easier to play the way you like to play and not try to force it or beat yourself up about it. That's not saying you shouldn't try new ways because those can be fun or even open you up to a new way to experience the game.
I find I go back to saves that I've put the most thought into creating the characters. I may have put them in homes that I feel fit their personality or is the right home for them. As @Metaphasic said above I usually go back if I emotionally connect to them on some level.
What do you love outside of the Sims? Can it be played out in your game? Or make an alter ego and do all the things you wouldn't normally do in real life. You can even cheat on your favorite save games and start a new one now and then. It's okay. You know you've made a good one when you go back to it.

I used to have the same issue with getting bored and starting over.
I found playing several generations on the same lot and house gets me bored quickly, so now I have the heir move out and start over on their own. The parents and one sibling are left behind in the family home. Depending on how much money the previous gen has, I give the heir 20% of that to build a small starter house and turn it into a family home as they earn more money.
I find doing this keeps me interested and my current save is on gen 7, and so far, I haven't been bored yet.