Ok - I'm vastly overshadowed by the majority on here but that does show a few things - bear with me:
I'm the same height but 78Kg and I've done my fair share of gymwork, before I started cycling. Your bench is def better than me and at the moment, you're not built for cycling. All that muscle on the top half is generally just dead weight you have to carry. If your legs are skinny (although 250kg isn't light) then they have to work hard to carry it around. Which is why, unless there was a massive headwind or a fair amount of climbing in that, 10.5mph for an hour is a fairly relaxed pace - so it would appear your legs get tired or can't output high for long. Again, bear with me...
I don't mean as a criticism - its an indication of where you are currently. Cycling for any sort of endurance is not about power but about sustainable power. If you're flagging after an hour, this is something that you need to develop. In doing so, you will burn more energy. Your asthma is obviously something that will impact this as your breathing will be important to supply oxygen but set yourself a reasonable goal to increase length of ride. Change the pace up and down, to work different parts - but keep in mind the slower recovery pace as it really benefits in the longer term and you can keep going for longer.
You will get there but it'll just take a bit of time and changing the way you workout. Aim for longer rides I think to get your metabolism up and burning for longer. There is a mass of knowledge and advice on here so take what you can from that too. And update on the progress too!