It’s fine.
Don’t allow yourself to become scrupulous over such practices. God doesn’t care how we pray in private prayer, as long as we pray. Sitting to pray is perfectly fine. If we are distracted by pain/discomfort we won’t be able to keep our minds on prayer but will be thinking about whatever is bothering our bodies. We can pray while driving, while walking, while riding a bike or doing housework, etc. Prayers is always good no matter how or where it is offered. Liturgical prayer, as in the Mass, has prescribed postures we are to use but even there, if it’s too painful to kneel, you are not obligated to do so. Sit if you feel pain when you kneel. God understands our physical limitations. You’re not doing anything wrong. 
I would point out that although this the material found in the linked Protestant website looks okay, it doesn’t address Catholic prayer practice, such as the differences between public, liturgical prayer and private prayer for Catholics believers. It contains one person’s interpretation of prayer postures, not what is prescribed for Catholic worship and private prayer.
No one is bound to the impossible.
I can’t kneel at mass at all … I have arthritis and tendonitis on my knees … we can only do what we can do. Sometimes I can’t even stand because of back issues … I figure God knows even better than I what is going on.
