67 yo endurance athlete ran a 10k 53:53 is sub 50 possible?

Yes, a sub-50 minute 10K is absolutely possible for a 67-year-old endurance athlete, though it will require a highly structured and deliberate training shift.

• Running a 53:53 means you are currently averaging a 5:23/km (8:40/mile) pace. 
• To break 50 minutes, you need to hit exactly 5:00/km (8:02/mile). 
• This requires a 7.2% increase in speed, which is a significant but realistic jump for an established endurance runner. 
• Your current time is already highly impressive—placing you well ahead of the average 10K finish time for men over 65 (which sits around 1:03:00) and putting you in a strong competitive bracket. 
• The Challenge vs. Your Advantage
• As an endurance athlete, you likely have a massive aerobic engine and excellent mental stamina. 
• However, the primary physiological barrier at age 67 is the natural decline in maximal heart rate and a gradual loss of fast-twitch muscle fibers. 
• To drop nearly 4 minutes, you don't need more mileage; you need to train your body to tolerate a higher sustainable intensity (lactate threshold) and improve your running economy. 
• How to Close the 3-Minute, 53-Second Gap
• Target the Lactate Threshold:
• The 10K is fundamentally a threshold race. 
• To run a sub-50, 
• incorporate weekly tempo runs. 
• Start with intervals like 3 x 2 km at your target 5:00/km pace with 90 seconds of jogging rest, eventually building up to a continuous 5–6 km tempo run at that target pace.
• Introduce Neuromuscular Speed (Strides):
• To fight off age-related decline in stride length, add 4 to 6 short strides (100-meter accelerations) to the end of your easy runs twice a week. 
• Focus on a quick turnover and tall posture to recruit those fast-twitch muscle fibers without overtaxing your aerobic system.
• Prioritize Heavy Strength Training:
• If you aren't lifting, this is your secret weapon. 
• Two days a week of lifting weights (focusing on squats, deadlifts, and calf raises) improves running economy—meaning your body will use less oxygen at that faster 5:00/km pace.
• Ruthless Recovery:
• Masters athletes can often train as hard as younger runners, but they cannot recover as quickly. 
• Keep your easy days truly easy, maximize sleep, and give yourself a slightly longer taper (10 to 14 days) before your goal race.