Based on no medical training whatsoever I would say if you had that sort of blow and are now 3-4 weeks later getting dizzy spells again then you need a very specific all clear before carrying on with your marathon plans for this year.
Personally I would forget getting a "couple" of twenty milers in. You have had an injury and you need to protect it to ensure you start. Easter weekend is probably last weekend that physically its worth getting a long run in anyway. So I would do one then and then taper.
If its your first marathon then its a bit of a toss up between not hitting the "wall" (so going slower than you are capable) against getting the best possible time (pushing it but risking blowing up)
If the 1:41 half was recent and you considered it a good race and achieved your potential then the predictors would probably say 3:35-3:40 for a marathon. That's faster than your 20 miler but not hugely so. You appear to have the miles in your legs and speed to go towards the bottom end of that range or possibly even a bit faster but theres the gamble you have to decide.
I always try and run consistently in a marathon - personally I have got best results that way.
Good luck - if your half is recent and 1:40 and was a good race - then I would say going out anything faster than 3:30 at absolute faster is optimistic and risks blowing up.Would say you are more likely to be 3:35/3:40 based on that half time.