When training for 180-200km bike segments, I've found the most useful intervals are 3x10minutes at lactate HR (or functional power) threshold with about 3 minutes rest (rest can vary a little if you like). That can be measured by HR or Watts/Power. LTHR can be set by a 30 minute time trial, where you ride as hard as you can for 30 minutes without stopping, and then take the last 20 minutes average HR as likely LTHR. With a power meter, it's even easier: just use the Normalized Power for 30-minute time trial, which is automatically provided by almost all Garmin devices, and is sometimes called FTP for functional threshold power.
The other useful "interval" can be done on longer brevets pretty easily: ride the last 20 minutes of every hour at tempo pace. Many Ironman world champions train this way. Tempo has a lot of different meanings in cycling, but in this case it usually means about 10bpm UNDER the LTHR--hard but maintainable. It can also vary more. So you ride 40 minutes at a moderate pace, then speed up to tempo the last 20 minutes, then repeat for several hours. It's difficult to go hard for twenty minutes the last few hours.
As we get older, hard anaerobic training becomes more difficult, but even a little helps. Yet an 81-year-old Ironman Hawaii finisher said he got anaerobic at least once a day in some sport.
Regards,
Tom Rodgers