It's astonishing really just how much the country has shrunk time-wise even since 1960. Looking at the timetables from that era, there appear to be no more than 5 or 6 departures from Paddington every hour in total. Departures to South Wales are much more in favour of the afternoon, presumably the assumption being people came down the previous day and stayed overnight, or used a sleeper?
Re getting to Brecon, looking at the pre war timetable there were more services, including one which got to Newport at 11. Services cut later and the connections never retimed perhaps?
It does seem to be the case that in the 'Beeching' era, the concept of connections wasn't seen as particularly important. Often stopping trains seemed to be timed apparently randomly and didn't always connect into fasts.
If you look at the 1965 timetable on Waterloo-Salisbury-Exeter for example, some slows connected, and some didn't. There are two-hourly express services, with the timetable not being that drastically different to that prevailing in the 80s.
However at that time, the line had many more small local stations than today, which were served by all-stations stoppers from Salisbury to Exeter, presumably WR DMUs.
The fasts left Salisbury at 1044 (0900 ex Waterloo), 1304 (this was ex Brighton, the 1100 ex Waterloo terminating at Salisbury presumably around 1244), 1444, 1644, 1854 and 2044. These generally called at most of the stations still open today, though tended to skip Tisbury and Crewkerne, and run fast from Sidmouth Junction (now Feniton) to Exeter.
The slows out of Salisbury towards Exeter were at: 0810, 1050, 1310, 1455, 1725, 1810 and 1955. It can be seen that while the first three provided good connections, the last three did not. The first at 0810 was too early for a Waterloo train anyhow.
I don't have any other concrete examples but have noticed this tendency when looking at old timetables from this era on timetableworld; local trains did not always make good connections with the fasts.