I can explain to you exactly why you are getting crushed by high leveled challengers.
The gym system is essentially a funnel, which you blocked up. You claimed the leader position for the first gym, then went inactive for a few days. What do you think everyone did while you were offline? They trained. The funnel began to fill up. Then when you got online and got challenged by people who had been training while waiting for you, the funnel was suddenly unblocked, and everything came rushing through.
If leaders, especially the earlier leaders such as yourself, are more active then you will have closer matches. If you stay active, I'm sure the level differences will decrease as trainers can start challenging you before they have time to build up to higher levels.