If I start running or doing cardio, will I lose all my strength?

This is a concern often voiced by people who are regular lifters. The short answer is it depends, but probably not. The demands of building strength and maintaining strength are quite different. If you are working on building strength/muscle and improving running at the same time your body will likely prioritize the cardiovascular and you will run into difficulty recovering from both high level stimuli at the same time. You can maintain strength and muscle while ramping up running by doing moderate difficulty lifts once or ideally twice a week without putting excess strain on your body. Long term lifters often neglect cardio/running and when they start, they are quite bad at it, which leaves them feeling exhausted. You will feel weaker if you are lifting while exhausted and not fully recovered from a run. It can be humbling to be worn out after a mile two run when you are used to lifting for an hour no problem. Cardio is important and distinct from lifting in terms of overall health and energy pathway used. So, to summarize, it is important to train both, when trying to meaningfully progress one area of fitness it is important to allow adequate recovery and not simultaneously try to progress another area.