If you believe you can make that BIG ONE happen, then you certainly have a much better chance than training for it, half-heartedly, or having an it might happen, but if it does don't expect miracles approach.
Yes you do need to monitor that weight loss figure and make sure it doesn't happen again, as it was a little high this week, but you're right not to panic and make hasty changes to training or diet to bring it down.
The worst thing anyone can do is have a week, where weight losses differ quite a bit it previous weeks, or weightlifting increases appear to be smaller or non-existant, because that can mess with someones head if they let it and then they revamp training and / or change diet and get more headjams, when what they tried doesn't correct things.
If your wife was alive today, she would be so proud of you, because after a long period of being "just some guy" physically, you took some initiative and committed to you. You are now living for YOU and realising more that YOU are really important and YOU do matter.
You ARE a good man Edgar and you are now being good to YOURSELF and it's good to see someone getting more out of life, by trying, instead of living in ignorance or living a deluded "you could get hit by a bus tomorrow mentality", to justify a terrible lifestyle.
YES it will be hard and YES it will ask questions of you and make you wonder if you're doing the right thing, but you truly believe you have what it takes to run a full one, not just a half one, then I don't you would.
Besides, would it be too much of an ego kicker if you actually started a full and about 2/3 of the way round had to veer off and finish early?
Perhaps, but at least you'd know you were good for more than a half and with more time, could be someone that could run a full on in whole, not in part.
You do have to truly feel it in yourself though. You can talk about a full all day long, but if it doesn't feel 100% right for you, then you'll be wasting your time.
With more time and training, for the whole endeavour to sink in more mentally, you will know how you feel whther it's the same or different. Personally, given what you have already acheived, I'd back you to do it, and actually finish, I really think you are that guy.
This could also be something you do for your wife too. Imagining her being there to watch you cross the line, running it in her name. Not to sound maudlin, but if that wasn't a solid reason to do it and believe, what would be?
You're doing great right now, so just keep believing in yourself and keep taking it a day at a time. Whatever happens, in the past few months you have surely given yourself a better future, so be proud of YOU and don't stop believing in this guy, because he isn't that other guy, he's someone new, more committed, self believing and capable and he is becoming a man that can be a rolemodel to others, for his attitude and willingness to change and make good.