How do I build work relationships that last for years?

You build relationships that last, and over the years they become something solid you can lean on. You did not do that by working an angle. You did it by being someone worth knowing over a long time, and that reputation now travels ahead of you. The thing to protect is that the trust stays real and never turns into leverage you cash in. The moment people sense they are being used, decades of goodwill can thin out fast. Keep giving more than you take, especially now that you could get away with taking. That restraint is what keeps the whole thing sound. Think of one person in your network who can do nothing for you right now. Do something for them this week anyway. That is how you know the relationships are real and not a ledger. Tend them like that and they will hold for life.