Limits on flat bar bike

I bought a flat bar bike, giant rapid 4, on BTW a couple of years ago with a view to commuting. Duly left in the shed until about a month ago when I started doing evening and weekend spins round where I live. Think I've got the bug now and want to extend my distance. I am currently doing a 30k loop 4/5times a week taking about an hour and twenty. Thing is I find that once I go over the hour I start to feel a bit numb and tingly in the hands, can't seem to find a comfortable grip position at all.
I have read elsewhere that drops may be a cure for this.
Planning to work out to 50k distances over next weeks so was wondering if I am reaching limitation of flat bar and should I change?

Wiggle currently have felt Z85 with shimano 105 kit for €852. Seems like this would be suitable for old timer like me. Is now the time to jump or should I stick with flatbar till new year,?

Rob