Trying To Break Plateau, Went on Ultra-Low Carb Diet, What Next?

I would appreciate suggestions as to how I should proceed.

In brief - I went from 214 lb and 36" pants a few years ago, to 180 lb and 32" pants last summer.  My goal is 170-175 lb. I'm 50 y/o, 5' 11".  At 30 y/o I was 162 lb.  I don't think it is realistic to be that light again, but something about 10 lbs heavier than my 30 y/o weight seems a nice goal.  

When I got down to 180 lb, last summer, I was riding my bike about 100 miles a week, went on a few century (100 mile/day) rides, was swimming, generally pretty active.  But I got "stuck" at 180 lb, well I had one day in the 179s, but basically I couldn't get below 180 lb.  Rather than get frustrated, I decided to just chill for a while and maintain there. Err, maintenance didn't work so well, as winter came I apparently went into hibernation fattening mode and my weight went up to 190 lb by last Christmas.  

This January, I decided to get back down to 180 lb, but it just didn't work.  Since January, I'd been eating average 1,600-1,700 cal/day (I log my food via the Calorie Count iPhone app).  My main exercise is commuting by bike and doing most of my errands by bike too, cycling average 25 min/day at a good pace, which supposedly burns about 300 cal.  I occasionally go to the gym for a hour's spin class, and occasionally run 3 miles (take the bus to work and run home).  At my weight, it seems like I should have a pretty hefty caloric deficit going.  As far as my diet goes, I eat a lot of fruit (apples, bananas, pears), vegetables (root vegetables like beets, rutabagas, radishes, also greens, squash, salads), meat (beef, pork), poultry, and fish.  I scratch cook almost everything and almost never eat restaurant food or prepared/packaged food.  I eat very little potato or rice, almost no bread (we've gone mostly gluten-free), never eat sweets, pastries, soda, etc.  But despite what should be a caloric deficit and seems like a healthy diet, my weight stayed stubbornly in the high 180s.  

Now I was getting frustrated.  And increasingly feeling that weight loss is more complicated than simply "calories in vs calories out".     

Enter my friend, a guy about my age who is pretty into exercise and weight control (he is 6'0", 180 lb) who suggested I try two weeks on an ultra-low carb diet: 1,600 cal/day but at most 20 grams carbs/day.  Okay, I've been on that diet for six days now.  Since this past Monday, I've been eating pretty much nothing but meat, poultry, fish, and eggs, with really minimal amounts of anything else.  Plus a daily fistful of vitamin and mineral supplements.  Still doing the daily bike commuting. 

I have another 8 days left on this ultra-low carb diet.  At this point I have "lost" 6 lb - was 187 lb Monday morning, was 181 lb now.  I realize the great majority of this loss is water weight, which will come back.  But I hope that by the end of the two weeks, which is Monday after next, I'll have legitimately lost at least a few pounds.   

My question is: what should I do after this ultra-low carb period?  

Should I:

a) go back to what I was doing before (avg 1,600-1,700 cal/day, basically meat/poultry/veg/fruit with very little grains/sugar) and hope that my body will respond now that its been spanked with two weeks of nearly-no-carb?

b) go on a low-carb diet, not <20 grams carb/day but something more like 50 or 100?

c) alternate a) and b), to keep my body confused?

d) something else?

I know I can't stay on the current ultra-low carb diet for too long.  It doesn't seem very healthy, and I'm noticeably weaker on the bike, which is no good as I'm preparing for a 200 mile ride in July.

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