I'm closing in on 40 and trying to run my first sub 4 hour marathon before then. I figure you don't really start running that fast after 40, so this is my last chance to ever be a 4 hour marathoner.
FWIW - I started running distance seriously only when I was 38. Ran my first half at 39 and ran my fastest half yet ( 1:39) at 44. Ran my first marathon at 45 - (3:56). I've gotten faster since then (per my 5K speed), but haven't run another marathon since - it is a significant time commitment.
You have more time than you think, especially at the marathon distance. If you were looking for sub-3, then yeah, you’d better hurry, but 40s isn’t at all old for a sub-4 marathon, especially if you’re willing to put in the miles during training.
Males have to get to 65 years old before a 4-hour marathon will even qualify them for Boston.
I'd wanted to qualify for Boston in my 20s, which at the time required a 3:10 marathon. It was just barely at the limit of possibility, if I had a perfect day, which I never did.
Then they dropped that limit to 3:05, and that might as well be on the far side of the moon. So, I never run Boston. Ah, well. That was a very stretchy stretch goal.
Don't sell yourself short. I only started running when I turned 38. I ran my first marathon in 3:53 when I was 42, and my second one in 3:41 when I was 44. I'm now nearly 48 and am planning on doing another one later this year and am training for a sub 3:30 time.