The very first thing that strikes me is: are you sure the treadmill belt is sufficiently tight? You mention it's been gathering dust for years/etc, which means it may well not be. This has a significant chance of making your steps more unsteady (as the belt itself may slip or be unsteady) which might account for that "stutter".
You SHOULD be able to tighten it yourself (we have a hex key to do so), but depending on how old it is that might take some searching on the internet for a manual (assuming you don't have the manual anymore).
The second thing is that I'd recommend maybe slowing down for a bit first. Yes, you have stamina built up already for swimming (yay!) but you're actually using different muscles when you walk! Especially around stability in your core, in your legs and in your hips. There's all kinds of SMALL muscles that are now suddenly being asked to hold you upright that are used to being supported by the water (that's part of why water is such a low-impact cardio!), and that will impact EVERYTHING.
If you slow down but focus on your posture and form while you walk rather than on I MUST WALK FASTER, you'll still be getting benefit, but also be also allowing the REST of your body to adjust to this new position and activity. It'll also let your body potentially learn how not to stumble on the front of the machine, allowing you to fix up your posture a bit.
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You SHOULD be able to tighten it yourself (we have a hex key to do so), but depending on how old it is that might take some searching on the internet for a manual (assuming you don't have the manual anymore).
The second thing is that I'd recommend maybe slowing down for a bit first. Yes, you have stamina built up already for swimming (yay!) but you're actually using different muscles when you walk! Especially around stability in your core, in your legs and in your hips. There's all kinds of SMALL muscles that are now suddenly being asked to hold you upright that are used to being supported by the water (that's part of why water is such a low-impact cardio!), and that will impact EVERYTHING.
If you slow down but focus on your posture and form while you walk rather than on I MUST WALK FASTER, you'll still be getting benefit, but also be also allowing the REST of your body to adjust to this new position and activity. It'll also let your body potentially learn how not to stumble on the front of the machine, allowing you to fix up your posture a bit.