I have a 17 mile commute to work, so 34 mile round trip, mostly flat but kinda downhill going to, kinda uphill going home. I commute 2-3 times/week if it’s not stupid cold or wet/ snowy. Takes me about 1:05 in, 1:15 home. I’ll be 48 this August, so I’d say that at 12 mph you’re doing fine. I try and try to reduce my times (increase overall speed) but about the only times I’m ever noticeably faster is when I have a consistent tailwind! As someone else said “my speed is my speed” so don’t stress about it. Rather, enjoy the ride and get there when you get there...