I can see that many commenters are men.... I have been there, in the situation of your girlfriend and yourself, and can tell you what helps:
Do not tell her directly how you feel, no matter what the other blokes here say: she is already feeling self-conscious about her weight, and probably is already thinking whether you find her attractive anymore. If your sex-life has also dwindled, she will probably be blaming herself and her body for it - and that's probably lowering her own sex-drive even more (I'm just assuming it's lowering, because that's usually the side effect of gaining weight and not exercising). All this is already ingredients to develop a terrible, long-lasting depression or eating disorder, if you fuel those thoughts by telling her directly and hammering her last remaining self-esteem down.
The
worst thing that a girlfriend can experience, is the nagging doubt,
that her boyfriend finds her unattractive, whether physically or spiritually/mentally. If you would tell her that you don't respect how she's not looking after herself, she'd think
a) "I'm obviously unattractive to him nowadays, I wonder if this is what he thinks when we cuddle/have sex?",
b) "If he's been thinking all that while he's with me, what else is he hiding or not telling me? Does he no longer truly want me? Is he thinking other women when he's with me in bed?",
c) "He obviously does not respect me. Can he really love me if he doesn't respect me?"
To avoid all these thoughts, I'd say that you should simply continue to show your example of a healthy lifestyle, and encourage to start new hobbies together: hiking, biking, swimming, tennis, whatever rocks your boat. Tell her, how important it is to you to share outdoors and activities together with your soulmate, and
how you'd like her to be part of what you most love in life after her: sports, health and an active lifestyle! It's about sharing life and the things you love together! To spend more quality time with her!
Inspire her by describing how exercise, healthy lifestyle and outdoors make you feel about life: lifted, inspired, energetic! Tell her how much you love life which is active, and how bad it makes
you feel when you haven't exercised for long. But don't comment on
her lifestyle, simply talk about
yourself. Don't even
hint at her! And if she reacts to you when you talk about all that - you know, she might go "I know... I should also... But..." then still avoid criticising her: rather propose to her: "well! Why don't we go and do something fun together? Hike?"
Once she goes for a single, fun, active hike, she will instantly feel lifted. The desire to exercise increases only after exercise. Get her out with you once for a nice, long, sweaty hike on a hill: enjoy kissing on the top of the hill, sit and watch views, smell the flowers. Get her sweaty without her even noticing. By
doing without consciously focusing on exercise, the simple activity will remind her how wonderful it feels to be fit!
Get her inspired by doing, not only discreetly talking!...I'm a girl who's always enjoyed being active and healthy, but hates routine exercise. I've also had eating disorders when younger, and was rather traumatised by that.
Over the years I have been rather healthy and active, but when my boyfriend met me, I was nevertheless the blumpest I had been in years (10kg overweight), due to hectic work, poor weather (I don't like indoor exercise very much), etc. etc.
...
But he did exactly as described above, and was very gentle. I never even thought back then, that he might have been concerned about my weight --
Because I felt truly comfortable, desired, respected, admired and loved by him.
He managed to inspire me back into an active lifestyle. Now looking back at this, I am actually certain he was, in his subtle way, consciously inspiring me to do this, while avoiding direct criticism. But I'm not feeling offended now, when thinking about how he did it: on the contrary,
I am grateful he brought the best out in me and in himself, without letting his own impatience and sense of superiority to take control... and made me feel accepted as I am, yet inspiring me to want to go back to a healthy lifestyle.
Hope those thoughts help. Good luck and keep us posted how it's going!