TLDR: Zone 2 works. It's just not the only thing that works. Higher intensities give you adaptations zone 2 can't; one or two conditioning sessions a week won't destroy your muscle; higher work capacity from conditioning will help you get bigger and stronger. Your casual walk around the block isn't zone 2 training.
Everyone's on the zone 2 bandwagon. Yes, it works and has its place. 30 to 60 minutes at a pace where you can talk but you'd rather not builds mitochondria, capillaries, fat oxidation, all of this makes you recover better.
But it seems like it's become this one answer for everything. And most people aren't even doing it right. Zone 2 is roughly 60 to 70% of your heart rate reserve. Walking around the neighbourhood at resting heart rate plus 20 beats is zone 1. It's good for your general health, but it's not a training stimulus. 10,000 steps of zone 1 doesn't equal one zone 2 session. The intensity has to be there.
Zone 2 never touches your lactate threshold, so it can't raise it. Helgerud et al. (2007) matched four groups for total work and had them train 3 times a week for 8 weeks. The 4x4-minute interval group at 90-95% max heart rate improved VO2max by 7.2%. The long slow distance group at 70%? Significantly less. Same work volume, different result.
Zone 2 also barely recruits fast-twitch fibres, doesn't build up enough hydrogen ions and lactate to train your buffering capacity, and doesn't push lactate threshold (the exercise intensity at which your body produces more lactate than it can clear out)
If you play soccer, hockey, rugby, or anything with repeated hard efforts, zone 2 alone won't give you the conditioning adaptations you need on the field or ice.
And no, you're not going to lose your muscle. The interference effect is real, but 1-2 conditioning sessions a week won't make you lose all your muscle. Schumann et al. (2022) reviewed 43 studies and found no meaningful interference for max strength or muscle size. The only thing that took a small hit was explosive strength at higher aerobic volumes. One or two sessions a week is nowhere near that.
In a prep phase, conditioning is what lets you handle more training. Better aerobic capacity means faster recovery between sets, sessions, and weeks. More volume, more total work, and that's what drives strength and size.
The athlete with a larger base can handle more volume and work than the guy gassed halfway through his 3rd set of squats.
Train all 3 systems
Your lifting hits the alactic system, phosphocreatine and short max efforts.
Conditioning intervals hit the lactic system, that 30-second to 2-minute range where you're producing and clearing lactate.
Zone 2 hits the aerobic system.
So do zone 2. Don't do only zone 2; don't pretend a leisure walk is training; add one interval session a week; keep lifting; and stop stressing about losing muscle from 20 minutes on an assault bike.
EXAMPLE CONDITIONING SESSIONS FOR BEGINNER:
Sled Session:
Warm-up on bike or walk on treadmill, start 3 mins easy (RPE 3-4) then 3 mins moderate (RPE 5-6)
Anaerobic Conditioning: (sled push): 6-8 x 20 seconds at RPE 7-8. Rest 80 seconds walking between reps. Ratio 1:4 (work:rest)
Assault Bike Session:
Warm-up: 5 minutes easy, building to moderate, then 2 x 10 seconds at RPE 6-7 at the end of 5 minutes. 30 seconds between the 10-second RPE 6-7 efforts
Anaerobic Conditioning: 4-5 x 60 seconds at RPE 7, hard but controlled. Rest 3 minutes, still using the assault bike (RPE 2-3) between reps. Ratio 1:3. (work:rest)
Cooldown: 5 minutes Assault bike (RPE 2-3) or easy walk for 5 minutes
References:
Haff, G. G., & Triplett, N. T. (Eds.). (2016). Essentials of strength training and conditioning (4th ed.). Human Kinetics.
Helgerud, J., Høydal, K., Wang, E., Karlsen, T., Berg, P., Bjerkaas, M., Simonsen, T., Helgesen, C., Hjorth, N., Bach, R., & Hoff, J. (2007). Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine & Science in Sports & Exercise, 39(4), 665–671. https://doi.org/10.1249/mss.0b013e3180304570
Schumann, M., Feuerbacher, J. F., Sünkeler, M., Freitag, N., Rønnestad, B. R., Doma, K., & Lundberg, T. R. (2022). Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: An updated systematic review and meta-analysis. Sports Medicine, 52(3), 601–612. https://doi.org/10.1007/s40279-021-01587-7