I don't know if this is the "correct" way to think about it, but in a buy-and-hold situation, I think of the cash return as similar to the dividends on another investment, with the appreciation and rent increases as similar to market movements with other investments. So, thinking about the likely total return-- including appreciation, rent increase, equity accrual, tax benefits from depreciation and mortgage interest, etc - is the main number I would look at in this situation. With a mortgaged purchase, this is especially important, because only a portion of the worth is tied up in the equity, but you get to keep 100% of the equity gains over the purchase price. Tough to beat that outside Real Estate as a private, passive investor (although I would be glad to hear other strategies). Meanwhile, inflation is reducing your real costs (with the mortgage payments) while rents will keep up or exceed inflation.
Then, I would also consider how actively you want to be involved at any age, as the previous commenter mentioned in the 60-yo scenario. If you are personally managing it, you may want to invest in something more passive; if not, the management fees are eating into your returns.
Either way, this sounds like a great situation to be in. If this is your real situation, congratulations on your financial success and freedom!