Meditation is a journey, not a marathon race. It is very important to be comfortable. There is no need to pressure yourself to reach a quota of time.
The sitting position is important. If your legs are going numb then the first thing you must do is adjust the way you are sitting. It is likely that your knees are higher than your hips. This will create issues in circulation, the ability to have the back straight, and your breathing.
I suggest using a proper zafu and zabuton cushion. The zafu should be placed just under the sitz bones to lift the hips. The zabuton cushions the sensitive areas of the legs.
Draw one heal in to the area of the perineum and place the other just in front. The shins should roll slightly forward and the knees should then have greater ease is spreading open to the sides.
Watch me demonstrate this in this video:
If you do not have cushions, you can try using towels and rolling one towel as a bolster for under your sitz bones.
Be sure to stretch and loosen the body before attempting to sit for long periods of time. Listen to your body, it will tell you when it has had enough!
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My favorite Zafu and Zabuton style combination is a Zafu with side arms to support the thighs and a nice comfy Zabuton. Try one from Sage meditation cushions.