The influence was what the people who learned computing in the 70s on PDP-11s at university, went on to do with what they'd learned in the 80s.
Also, the 6809 shipped in 1978. Little known fact, the Macintosh was going to be based on the 6809 and Jef Raskin's team had 6809 Mac prototypes running before Steve got Motorola to drop their price on the 68000 (in part because Moto was panicked over losing out on the PC).
For sure, by volume the impact of the PDP-11 was almost entirely through micros and workstations because they were taking over so fast. Although, Unix and C did spread like wildfire from the PDP-11 to other minis. In fact, C was originally created for porting Unix.
In terms of timing, the PDP-11 shipped in early 1970 and was an immediate best-seller. Back then, students didn't get much hands-on computer time until their final two years so by the mid-70s its influence was already being felt. The 6809 was being designed in 1975 and the architecture was clearly inspired by the PDP-11's orthogonal ISA, interrupts stack handling, etc.
Also, the 6809 shipped in 1978. Little known fact, the Macintosh was going to be based on the 6809 and Jef Raskin's team had 6809 Mac prototypes running before Steve got Motorola to drop their price on the 68000 (in part because Moto was panicked over losing out on the PC).