James, like you I am 69 and will turn 70 in May. Started detecting in March of 1965. I've been dealing with a number of getting-old health issues, and that's been compounded by a bad spine [size=small](top-to-tail)[/size] and four specific injuries that have combined to make life miserable and really cut in on my detecting enjoyment. I'm not supposed to lift more than 5 to 10 pounds, and the neurosurgeon said the top limit is 8 pounds, and even that can be a struggle which is basically a gallon of milk.
Over the past dozen years I have had to periodically trim my personal detector outfit and go to lighter weight and more comfortable detectors. I can handle the Anfibio Multi, but with the 11" DD it is for much shorter durations that I would care to admit. I have found the round 7" Concentric coil to be just about the perfect match to provide me with in-the-field performance yet be more comfortable, controllable and well balanced. I go to the 5" DD in the densest debris sites.
My suggestions would be to make sure you are not extending the search coil way out in front of you, and don't be making big, broad sweeps from side-to-side. Work the coil maybe 12" to 18" in front of your leading foot, and I suggest limiting the side-to-side sweep distance to about 24" to maybe 30" or at the most 36". to 40", maximum, in very open areas. Running with broad sweeps and long rod extensions is one of the bad moves folks make that adds to the front-end / nose-heavy feel. It also induces faster sweep speeds and that can also be fatiguing, not to mention diminishing performance afield. I use the7" Concentric 90% of the time on the Anfibio and that helps make it more comfortable.
To be sure, I / we can't hang in there for very long with heavier, or poorly balanced, detectors like we got to enjoy way back in those earlier days. If you took the system box off the Anfibio you should have noticed it felt lighter, and if you had the 7" Concentric it would really be apparent. Unfortunately, with progress comes change and, in the case of the Multi-Kruzer or the Anfibio series, everything is crammed in the control system box, including the Lithium battery. And to make them waterproof to the depth they are good for, it requires a thicker [size=small](thus heavier)[/size] plastic housing material. And since all of that is perched atop the handgrip, it can give it a somewhat heavier feel and balance.
I have hunting buddies who have checked out the Anfibio Multi and they have all been very impressed and, with any coil mounted, they say it is well balanced and not too heavy or awkward. They are all 16 to 32 years younger than me, too, and in much healthier and stronger condition and don't notice things like I do. Weight is one thing but balance can sure make a difference. I have been trimming my entire Regular-Use Detector Outfit the past several months, letting go a lot of competitor's detecto9rs that were too heavy or too awkward and tiresome. My Anfibio Multi might be about the heaviest model I have left [size=small](other than some older models I have on-hand for use in seminars I do)[/size] but with a proper rod length and using the 7" Coil I can still get along okay for a while.
The bulk of my hunting is in very dense iron contaminated places and the best detector I own for taking on those tough challenges are the Nokta FORS CoRe and FORS Relic. Hanging on the end of a measuring scale with either their smallest coil or the 5X9½ open-frame DD coils, they might appear to be heavy. However, they have one of the absolute best balanced physical designs, and as impaired as my mobility is, in the first several days of last week I put in reasonably long hunt-times, for me, because they are so well-balanced.
Maybe check how you're using it and, if you can, try the7" Concentric coil to see how that might help you out.
Monte